I think this will become a tug of war between the "**** PC and anyone who wants it" crowd and the "I'm marginalized so I'll kick you in the guts" crowd, and will have very little to do with the dress.
I guess for me, I err on the side of not being an asshole, when it comes to things people say is hurtful, even if I don't understand. The girl rocked the dress, but she would have rocked any dress, she's smoking...hehe...ah to be 18 again. But if she knows that dressing up in the traditional cloths of another culture, then she chose to be a smokin' hot asshole...and if she didn't, but does now, then it costs her nothing to say "ah, damn, sorry, I didn't mean it like that, thanks for helping me avoid being an asshole in the future".
Now, I know the "**** PC" crowd will have a seizure reading that, but this is a debate forum, not an echo chamber, so I assume you're looking for different opinions, that's mine. Is she the devil? No. Is she an asshole? Maybe... Probably, though, she's just being 18... It's a good conversation to have, but I doubt any minds will change as a result, on either side of the debate.