Is there any mention of the opinion of freckles in ancient times? Or things like birthmarks, moles, etc? How were they viewed in their beauty standards?
Well one of the examples and mentions of moles for instance that comes to mind is Helen of Sparta. She is mentioned to have a beauty mark aka a mole on her forehead according to the latin-spoken source of Dares the Phrygius
Helenam similem illis, formosam, animi simplicis, blandam, cruribus optimis, notam inter duo supercilia habentem, ore pusillo
Helen was similar to them (Dioscuri), she was very beautiful, ingenious of mind, soothing, with the best legs, she had a mole between her two brows and she had a little, charming mouth
So as you see in this context it was actually presented as a positive trait for her appearance. Although we do not seem to see people for instance drawing these beauty marks on their face (as it happened to 18th century for instance) but it seems that they were considered a beautiful feature and in a way we still do. Moles back in the day as well as now were technically called "olives" because of their color and shape resemblance to olives and they are very common in Greece and I would expect in other parts of the world in general and Europe in particular.
In ancient Greek legends there is also the presence of a mythical seer called Μελάμπους (Melampus) that is also mentioned in the Odyssey as the one who taught Theoclymenus, the prophet that Telemachus saved and took with him who also in turn saved him from the trap of the suitors, as well as in legends such as the ones provided by Herodotus and speaks on a man from Pylos that claimed could speak to animals. He was the first, according to Herodotus, to teach people about Dionysus and his worship and gained prophetic abilities in Egypt.
One of the words attributed to him according to the legend is a book called "Περί ελαιών του σώματος" or "On the olives of the body" in which text supposedly we have a series of predictions on what the moles on one's body mean
The texts seem to be from a text called Scriptores Physiognomiae Veteres attributed to Aristotle unless I am gravely mistaken:
Ἐὰν ἐστὶν (ἐλαία) εἰς τὸ μέτωπον τοῦ ἀνδρὸς, πολλῶν ἀγαθῶν ἔσται κύριος. Εἰ δὲ εἰς τὸ μέτωπον τῆς γυναικὸς, ἡ γυνὴ βασιλεύσει, ἤ μεγίστη ἔσται.
If the mole is on the forehead of a man, he will be master of many goods. If it is on the forehead of the woman, the woman will reign or she will be the greatest
Ἐὰν ἔξω τῶν ὀφρύων τοῦ ἀνδρὸς, λαμβάνει γυναῖκα ἀγαθὴν, καὶ εὔμορφον· εἰ δὲ γυναικὸς, καὶ ἔστιν ἡ χροιὰ αὐτῆς πυῤῥὰ, λαμβάνει ἄνδρα πλούσιον, καὶ εὔμορφον. Εἰ δὲ ἐπὶ τῶν ὀφρύων τοῦ ἀνδρὸς, οὐ δεῖ συζυγῆναι αὐτὸν, ὅτι πέντε γυναῖκες ἀνὴρ γενήσεται. Τὸ αὐτὸ καὶ ἐπὶ τῆς γυναικός.
If it is outside the eyebrows of a man, he will get a good and beautiful wife. If the woman's body is ruddy of color she will get a wealthy and beautiful husband. However if the mole is on the brow of the man then no one should marry him because he will be the husband of five wives. The same goes for the woman.
Ἐὰν ᾖ εἰς τὴν ῥίνα τοῦ ἀνδρὸς, καὶ ἡ χροιὰ αὐτοῦ ἔσται πυῤῥὰ, ἀκόρεστος ἔσται τῆς συνουσίας, ὅτε καὶ εἰς τὸ κρυπτὸν ἔχει ἐλαίαν. Ἐὰν ᾖ εἰς τὴν ῥίνα ἤ εἰς τὸν ὀφθαλμὸν τῆς γυναικὸς, ὁμοίως ἀποβήσεται τῷ ἀνδρὶ, ὅτι καὶ αὐτὴ εἰς τὸ κρυπτὸν ἐλαίαν ἔχει. Ἐὰν εἰς τὸ πλάγιον τῆς ῥινὸς τοῦ ἀνδρὸς, ξενιτεύσει ἀπὸ χώρας εἰς χώραν. Ἐπὶ δὲ γυναικὸς, κακοποδίνα γενήσεται, ὅτι καὶ εἰς κρυπτὸν ἐλαίαν ἔχει.
If the mole is on the nose of the man and his skintone is ruddy then he is insatiable for lovemaking especially if the woman has a mole at her private parts. If the woman has a mole on her nose or the eye then she will be the same for her own husband especially if she has a mole at her private parts as well. If the mole is at the side of the nose of the man, he will be immigrating from country to country. If it is on the nose of the woman, she will have smelly feet and if she has a mole at her privates
Ἐὰν εἰς τὸ μάγουλον τοῦ ἀνδρὸς, πλούσιος γενήσεται. Ἐπὶ δὲ γυναικὸς, ἐὰν εἰς τὸ κάτω μέρος τοῦ σιαγόνος, ἐμπαθὴς ἔσται, ὅτι καὶ εἰς τὸ ὑπογάστριον ἔχει.
If the mole is at the cheek of the man, he will be rich. If a woman has a mole to her cheek or at the bottom of her chin, she will be emotional, the same if she has one to the bottom of her belly.
Ἐὰν εἰς τὴν γλῶσσαν τοῦ ἀνδρὸς, λήψεται γυναῖκα πλουσίαν, καὶ καλήν.
If the mole is on the tongue of the man, he will get a rich and beautiful wife
Ἐὰν εἰς τὰ χείλη τοῦ ἀνδρὸς ἐλαία ᾖ, πολύφαγος ἔσται. Ὁμοίως καὶ ἐπὶ γυναικῶν.
If the man has a mole on his lips, he will be gluttonous for food. The same goes for the woman
Ἐὰν εἰς τὸν πώγωνα ἔχει ὁ ἀνὴρ ἐλαίαν, πλούσιος ἔσται χρυσίῳ καὶ ἀργυρίῳ. Τὸ αὐτὸ καὶ ἐπὶ γυναικὸς, ὅτι καὶ εἰς τὸν σπλῆνα ἔχει.
If the man has a mole on his chin, he will be rich in gold and silver. The same goes for the woman as well as if she has one on her spleen
Ἐὰν εἰς τὰ ὦτα ἔχει ὁ ἀνὴρ, πλούσιος καὶ εὐλογημένος ἔσται. Τὸ αὐτὸ καὶ ἐπὶ γυναικὸς, ὅτι καὶ εἰς τὸν μηρὸν ἐλαίαν ἔχει.
If the man has a mole on his ears he will be blessed and rich. The same for the woman as well as if she has one on her thigh
Ἐὰν εἰς τὸν τράχηλον ἔχει ὁ ἀνὴρ, πλούσιος ἔσται σφόδρα. Ὁμοίως καὶ ἐπὶ γυναικὸς.
If the man has a mole on his throat he will be insanely rich. The same with the woman
Ἐὰν εἰς τὸν αὐχένα ἔχει ὀπίσω ὁ ἀνὴρ, ἀποκεφαλίζεται.
If a man has a mole on his nape, he will be beheaded
Ὲὰν εἰς τὰς ψύας, πτωχὸς ἔσται καὶ ἀτυχὴς βαστάζων γένους. Ἐπὶ γυναικὸς ὁμοίως.
If he has a mole on his loins he will be poor and unlucky in his children. The same goes for women
Ἐάν εἰς τοὺς ὤμους, δέσμιος ἔσται, καὶ θλιβόμενος.
If he has a mole to his shoulders, he will become a slave and unhappy
Ἐὰν εἰς τὴν μάλην, λήψεται γυναῖκα πλουσίαν καὶ καλὴν. Καὶ ἡ γυνὴ ὁμοίως.
If he has it in his armpit, he will take a wife beautiful and rich. The same with the woman
Ἐὰν εἰς τὰς χεῖρας, πολύτεκνος ἔσται. Καὶ ἡ γυνὴ ὁμοίως.
If he has them to his hands he will have many children. The same with the woman
Ἐὰν εἰς τὸ στῆθος, πένης ἔσται. Καὶ ἡ γυνὴ ὁμοίως.
If he has it on the chest, he will be a beggar. The same for the woman
Ἐὰν ἐπάνω τῆς καρδίας, πολυπόνηρος ἔσται. Τῇ δὲ γυναικὶ τοῦτο συμβαίνει, ὅταν εἰς τὸν μαστὸν ἐλαίαν ἔχει.
If he has it on his heart he will be miserable. The same happens to the woman and if she has the mole on her breast
Ἐὰν εἰς τὴν κοιλίαν, φάγοι ἀμφότεροι ἔσονται.
If they have it on the belly, both will be gluttonous
Ἐὰν εἰς τὸν σπλῆνα, ἐμπαθεῖς καὶ ἐπίνοσοι.
If they have it on the spleen, both will be emotional and sickly
Ἐὰν εἰς τὸ ὑπογάστριον, ἐμπαθεῖς ἔσονται.
If they have it to their underbelly, they will be emotional
Ἐὰν εἰς τὸ κρυπτὸν, ἄπληστοι ἔσονται ὑπὸ τῆς συνουσίας.
If they have it on their privates, they will be greedy for lovemaking
Ἐὰν εἰς τὸ φυσικὸν, ὁ μὲν ἀνὴρ ἀῤῥενόγυρος ἔσται· ἡ δὲ γυνὴ τοὐναντίον.
If it is on the penis, for the man it means he will have many male children while for the woman is the opposite
Ἐὰν εἰς τὸν μηρὸν, πλούσιοι ἔσονται.
If they have it on the thigh they will be rich
Ἐὰν εἰς τὰ γόνατα, λήψεται γυναῖκα πλουσίαν. Ἡ δὲ γυνὴ, εἰ μὲν εἰς τὸ δεξιὸν γόνυ, ἀγαθὴ ἔσται· εἰ δὲ εἰς τὸ ἀριστερὸν, πολύτεκνος ἔσται.
If he has it on the knees, he will have a rich wife. For the woman on the other hand if she has it on her right knee she will be good while if she has it on her left she will have many children
Ἐὰν εἰς τὸν ἀστράγαλον, ὁ ἀνὴρ (τὴν γυναῖκα) ἀπρόσωπον ποιεῖ ἀπὸ τῆς χιτωνίας· ἡ δὲ γυνὴ, τὸν ἄνδρα ἀπὸ τῆς ἐξουσίας.
If he has it on his ankle, the man will have a wife ugly that dresses well but for the woman that she will have a husband under her control
Ἐὰν δὲ εἰς τοὺς πόδας ἔχουσιν ἐλαίαν, πολύτεκνοι ἔσονται.
If they have a mole on their foot, they will have many children
Σημείωσαι οὖν τὸ ἐπὶ ἀνδρῶν καὶ γυναικῶν· ἐὰν ᾖ ἡ ἐλαία εἰς τὰ δεξιὰ μέρη, πλούσιοι καὶ ἀγαθοὶ πάντων γενήσονται· εἰ δὲ εἰς τὰ εὐώνυμα, ἐμπαθεῖς καὶ πτωχοὶ ἔσονται.
One must note that the differences between men and women, if the moles are on their right sides they will be rich and the best of all while on the opposite side they will be emotional and poor
So I would expect that as long as a mark was not overly covering some feature or other, then it was considered beautiful or daresay have some sort of interesting use for "mysticism" haha although as you see seems rather silly and I doubt you can find many people around here without quite a few moles on their bodies so it is most likely that many of these "predictions" contradict each other.
So the moles were just treated as something that is there quite frankly and as long as it wasn't ruining the analogous features then it was no real issue I suppose.
Now for freckles from a quick search I cannot find much but we have some mentions from people like Nicander (2nd century BC) in which he apparently mentions a treatment with mullein, havergrass, celandine, wild carrot, and the root of bryony to wipe off spots and freckles. Pliny also seems to add some treatments for that
So as far as freckles are concerned it seems that they were not preferable. On one hand it was a sign that someone was pale of skin, which was the beauty standard in Greece and Rome especially for women but on the other hand it is an alteration usually made by the sun so I would expect higher society ladies wouldn't want freckles on their face. Plutarch too seems to mention freckles that people tend to try and remove so I would say they didn't see them as the epitome of beauty for clear skin was much more preferable.
Nowadays I think we do see as a light powder of freckles on the face as charming but I would expect not to an excessive degree (I mean as a general average. Of course beauty is in the eye of the beholder and it is subjective)
This is the result of a very quick search of mine. Sorry if it is not enough for now.