Initiatives like Take Back Halloween and the innumerable blog posts about what women should and should not be for Halloween are extremely problematic. While they may not outright slut-shame women who decide on a sexy costume, there's certainly a tone. Besides, "women with imagination" don't generally need a guide to help them choose a Halloween costume. We had this on lock in August, okay?
For anyone that just wants to roll up to Target and buy something from a plastic bag and wear it with a pair of fishnets and call it a day, do it. You do not lack imagination, you are not slutty, and nothing about the costume you've purchased says anything about you other than you liked what you saw and you bought it. Halloween can be really hard and a good costume takes time and money. Last year I spent $160 to be a lion tamer, and this year my remarkably easy costume is already causing me anxiety and costing me money. I, too, hear the siren call of the prepackaged costume.
There's nothing wrong with looking sexy, ever, and I wish we lived in a society where men and women could look at a woman who is dressed sexily and keep whatever they're thinking to themselves. Recognize that a person dresses sexy because she wants to and because she put on what she put on, looked and the mirror and liked what she saw. If you don't appreciate it, just remember that it's none of your business anyway and shut the Hell up.