My opinion certainly won't be a popular one, but....
First of all, I don't agree with blaming the victim in cases of sexual assault and rape. Regardless of how a person dresses, no one has the right to touch them, and when it comes to sex everyone always has the right to say no.
However, going to a convention dressed in skimpy outfits where there are going to be a bunch of men, yeah, what do they expect? Some of them may be dressing just like the character, but can't they make it at least slightly more conservative? I don't think any woman should have a reasonable expectation of walking around with her cleavage hanging out and not having any man ogle her ever. Men are going to look, and they're probably going to comment. If you're the type to be offended by it, rethink your costume choice.
Let me come at this from another angle... I'm a heavy person, and I'm well aware of it. Let's say one day I decide to walk around in a tube top, showing off my fat rolls in all their glory. Even when I dress reasonably people stare and make comments, so I can only imagine how well that would go over. My feelings are hurt. I'd be willing to bet pretty much anything that most of you would believe it was my own stupid fault for going out like that knowing I'm fat and unattractive.
Should the rules be different for an attractive woman who wants to walk around like a two-cent slut? I think not. Even when I show cleavage and get stared at I know I don't have the right to complain when people stare. The obvious solution to that is to buy higher cut shirts. Touching them is crossing the line, cat calls... if you don't like it, cover up a little. It's common sense.