I understand mono's point to some extent.
If I was told "There is a serial killer right now who targets female with brown hair tied in a ponytail and blue clothes, in your area", I would make sure not to tie my hair in a ponytail and not to wear blue. And I assume women who don't have brown hair would avoid dying it.
The problem is when people assume all rapist are triggered by the same thing, and that this thing is sex-appeal. Rapists are attracted to people they feel vulnerable enough to assault, which is why a lot of victims are elderly, handicapped or young. They're not targeted for their looks (although I'm sure some of them might be drop-dead gorgeous) but because they're seen as easy targets.
Rape is, after all, a crime of power, with sex used as the weapon, not the end goal. The end goal is telling the person "I control you".
It's sad when people say "she shouldn't have dressed that way", because they wouldn't say that a guy wearing a suit is calling to get mugged ("he's wearing expensive clothes! Surely he should wear rags if he doesn't want people to steal his money!"). I think it's offensive to the women but also to men, because it's sending a message that normal men will become rapists if women around them are dressed a certain way, which is ridiculous. Rapist are rapist and not influenced by the way their victims dress any more than other criminals. And the reasons they target people (their vulnerability) is not something that's as easy to work on, short of hiring a bodyguard, which not everyone can do.