The same rules that apply for dressing for the club are what sexy Halloween costumes fall under. There is a line. If you're showing off your pussy, it's trashy. If your butt flops out, it's trashy. If it's form fitting, while leaving a bit for the imagination it is sexy.
I mean... it's Halloween. Does it really matter? The sexy girl scout is going to look just as ridiculous as the fully covered person in a gorilla suit. If you're going to a party, nothing is trashy (unless you're going as sexy duct tape and just cover your private areas lol). If you're going door to door with your kid though, it's better to be family friendly
Depends, if you're just hanging out or going trick or treating or something then def trashy. Maybe if you are going clubbing or something, then it could be sexy, but only in those specific circumstances.