It totally depends on how creative the costume is and in what context I see the girl.
If the party is in a place where sexy costumes are acceptable and she's not the only one dressed like that, I love sexy costumes. They can be a lot of fun! Of course, if the sexy costume comes out at a party with people from the office who shouldn't see you like that, it may be poor taste.
The costume also makes all the difference. If the girl just threw on her sluttiest black outfit and then threw a witch hat or cat ears on her head, I see a girl whose main objective was to be sexy, with the "costume" part of the outfit coming only as an afterthought. If you take some time to create a GOOD costume that happens to be sexy, I love it. Sexy cosplay or crossplay outfits are wonderful. I don't even mind costumes like "sexy giraffe" or "sexy watermelon" or any other sexualized costume you want to come up with, as long as you spend just as much time on the main costume idea as you do on the sexy bit.
For a costume party where sexy costumes are appropriate, I love a bit of sex when combined with a good effort at a creative costume. The store-bought "sexy costumes" like the sexy police officer and sexy maid show to me that your agenda was to be sexy, not to have a creative costume, and I don't love that. I'd save those costumes for the bedroom and think of something better for a costume party.
In short, sexy costumes are great if you want to wear them and you do so in the right situation. BAD costumes, sexy or otherwise, make me sad.