It seems to me that you and I view beauty differently. For starters, calling someone "ugly" because they don't fit established beauty standards or because they don't match what you consider attractive seems questionable to me.
No one is truly "ugly," and the mere thought of labeling someone as such seems superficial to me.
Severus's facial features are quite common, in fact, and are even considered part of a beauty standard in some parts of the world.
Crooked teeth are considered beautiful in Japan, for example, and even if they weren't, most people don't have perfectly straight, white teeth.
Hooked noses are beautiful! Period. There's nothing to argue about. (As someone who loves the arts, these types of noses create beautiful profiles, I love them.)
And the greasy hair—at least 1 in 4 people have greasy hair genetically—it's so common that I don't even see how it could affect whether someone is seen as "ugly" or "beautiful."
His skin... Are we really going to ignore that there was a time in history when looking sick and pale was a standard of beauty?
Beauty is subjective, and everyone has their own perspective on what is "beautiful" or "ugly".
"allowing Snape to be ugly" is something that bothers ME and will continue to bother me, because in my opinion, what it does is perpetuate harmful beauty standards and the idea that these features together (or separately) make someone ugly, or worse, deserving of being humiliated and mocked, as the books encourage throughout. It only normalizes hatred and mockery of people in real life who have these features.
Also, the image that people who constantly repeat "Snape's right to be ugly" usually have in their heads is a distorted, ridiculed, and exaggerated image of the character's features to generate mockery or disgust.
And this is a reality in the fandom. You just have to see the way people react to certain fanart or fancast, even those that meet all the canonical descriptions of the character, for some people they are not good because they aren't "ugly enough" (And it never will be, because the dehumanization that surrounds Snape as a character has created this image where, if the character does not disgust you when you see him, then he is not Snape)