As someone who isn’t particularly fond of Jon Snow as a character I do still wonder why there suddenly seems to be a consensus that Jon is “plain”. I suppose it just doesn’t make much sense to me that GRRM would write the son of two people who are considered handsome as not particularly attractive.
I’m not saying that he isn’t plain, but the basis for this assumption seems to be that Jon looks like Ned. Which he does, but “looks like” doensn’t mean exact copy, even in ASOIAF.
Jon obviously looks like Ned only because Lyanna did, and when Catelyn thinks that Jon looks like Ned, she seems to be mostly concerned with coloring and shape of face, which is something that Brandon, Ned, Lyanna and Benjen all seem to have had in common, and Brandon still seems to have been considered handsome, while Lyanna is said to have been beautiful.
All I can see that we know about Jon is that he is lean, probably of medium height, has the hair and eye colour and long face of the Starks, and a scar. And that he is currently dead. A wildling calls him pretty once, I believe, but that might just be because he is better groomed than they are. Other than that I can’t see that we really know, because he hasn’t been described by another POV that notices physical appearance (enter Sansa?).
I think that the argument runs more in the direction of “Jon Snow is not as good looking as Kit Harrington, the actor who was cast to play him” rather than Jon Snow is ‘plain’.
And because Kit is ridiculously good looking you don’t really need to drive home the point....
It’s more of a reaction to the argument that antis like to throw at Jonsas. “You just want Sansa to have a handsome husband, you superficial delulu twats.” Or another argument runs: “Jon can’t be the Beast to Sansa’s Beauty arc, because look at how ridiculously good looking he is, you imbecile teenage soccer moms.”
This is of course and exaggeration on my part but you get the gist.
Anyway, GRRM doesn’t give detailed descriptions anyway, in this you are right, but we see Jon through the eyes of Tyrion, who remarks that Jon has the Stark look, but does not call him handsome (and Tyrion realizes this sort of things, because he is jealous of handsome men), and Sam, who also would be inclined to remark on Jon’s looks due to his own awkwardness.
Melisandre is not prone to note men’s looks I would say. But the evidence strongly points towards Jon not being a stunner. Handsome men are noted in ASOIAF, Jaime, Satin, Renly, Loras.... They are called handsome. Jon is not called handsome. And some PoVs close to him might note.
So, if we think Jon might not be that handsome it is an “argumentum e silencio” (argued from the fact that nobody mentions he is handsome), safe Ygritte and she is certainly biased.
Ygritte helped pull him up. "He's bleeding like a butchered boar. Look what Orell did t' his sweet face." (ASOS, Jon II)
So, this seems to insinuate that his face is ruined by Orell.
That doesn’t mean that he looks bad or ugly, but he is no Satin or Jaime or Renly. That’s for sure.
Imho, you could call that ‘plain’ or ‘not extraordinary’ or whatever. And beautiful parents can have children that look plainer than them.
I’ve always taken Cat’s opinion that Jon looks like Ned at face value. And Ned does not look extraordinary. Cat loves him very much, but she was disappointed by his looks. And family resemblance is a strange thing. I have a nephew who looks exactly like me up to a point that you can only identify who is who on children’s photos because we dressed very differently in my childhood. He could pass for my son anytime, while my daughter looks like my husband’s sister.
So, I’ve always taken ‘Ned doesn’t look extraordinary’ as a hint towards Jon also being more on the plain side.