You didn't ask me, but I feel inclined to respond.
I think that attractive characters aren't a problem in themselves, but can be a symptom of another problem. If a character is really super totally hot, but their attractiveness doesn't have any interesting implications on their personality, it could be an indication that someone is playing for wish-fulfilment more so than to create a good story. Chances are, they're just playing an idealised version of themselves, whether they realise it or not. And that's usually detrimental to good story-telling.
For the second point, there's a notable difference between someone playing a character who is snarky teenager, and an actual snarky teenager playing themselves. The former person knows he's playing an annoying character, assumably with the intention of making them less annoying through character development. The latter person probably thinks the way they're acting is actually cool. Their perspective is limited due to their inexperience, and if they don't see the flaws in their own character, they're unlikely to create any interesting development.
These are generalisations, of course. Point is, the problem isn't about snarky teenage supermodels specifically, but about the fact that they are statistically likely to overlap with a miasma of bad roleplaying practices.
This is pretty much the reply I would have typed out last night hadn't it been 4AM at the time. I don't know whether to feel envious or grateful. I'm going with both.
I define Mary Sues not even necessarily as flawless characters, but those who flaws (if they have any) are not pertinent to their experiences throughout the story. Therefore they move through the entire RP having never been forced by their fears, shortcomings, failures, and unrealistically grand ambitions, to learn, forgive, mature, or cope.
If you've ever seen that one hyper-badass dude in an RP who is, somewhat bafflingly, "afraid of deep water," one of two outcomes is likely: the player is a fan of The Truman Show who inserted a little tribute to the film into his application, or more plausibly, he wrote a Mary Sue. He gave his character "flaws" as lip-service to the practices of good RP, without realizing that because that character will never have to sail across open water during the RP, it serves no purpose but to trick the app's readers into thinking he boasts depth and realism in his writing repertoire. The female version of this cliche is probably the "fear of spiders," again never confronted during the story, but the principle applies to any "character flaw" which is not relevant to the events happening within the RP's narrative.
It's simply inherent in the notion of "flawlessness," literal or practical though it may be, that your character will sport no physical flaws either. For the record, I've seen plenty of poorly written ugly characters, so deforming and marring a character isn't a one-way ticket to creating a more interesting personality by any means; but not once have I seen an ugly Mary Sue.
It's all about intention, really. You can play a good-looking character with hopes, dreams, ambitions, fears, and insecurities, just like those which exist in real life, because yes, people can be incidentally good-looking; or you can design them deliberately to be gorgeous supermodels, a single symptom of holding the same fucked priorities which spawn Mary Sues in the first place.