Because a lot of geek guys imagine (possibly correctly) that even though she's supposedly in their social clique, and thus ostensibly obtainable, that she wouldn't date them/fuck them. She'd date/fuck another really hot guy (or girl). And this makes them angry. So they pretend she must be some kind of faker to rationalize it away: she isn't really into the hobby (oh horseshit, those costumes take hundreds of hours), she's only doing it for the attention (so what if she is? so are a fuckton of fit male cosplayers - where's your rage at them?), she's not _really_ that hot, anyway (ahahahaah you wouldn't be wasting your time commenting if she wasn't, bro).
And a lot of geek girls are simply jealous of her attractiveness, in the same way many women snipe at overtly sexual actresses/porn stars, and the same way men snipe at boy bands and pretty boy actors. Someone of the opposite sex you're interested in probably wants to fuck her/him more than they want to fuck you. It hurts. So, try to tear her down with rationalizations of their own: she has implants (honestly, she probably does, but so what? you could have them too), she wears a metric fuckton of makeup (so do just about all professional cosplayers, not to mention models, so again - so what? you could too), she uses photoshop (her and every other major cosplayer on the planet, some much, much more overtly than she does).
It's an ugly side of humanity, but at least she'll never hurt for press, and people will continue to consume her media whether they hate her or not, because she's genuinely, undeniably talented at what she does.