My first thought is to be appalled.
On the other hand, it occurs to me that I really don't want my child to be a ghost, or a vampire, or Richard Nixon, or ... well, you get the idea. So why would I not object to them wearing a ghost costume, or a vampire costume, or a Richard Nixon mask?
On the other hand, there's enough sexualizing of kids these days that having a kid dressed up as a pimp is kind of bothersome.
But I wouldn't object to an axe murderer.
But I would object to a bloody soldier.
But I wouldn't object to an unbloody soldier.
But I would object to a stripper.
But I wouldn't object to a drug dealer.
But I would object to a drug addict.
But ...
... I think I keep Katherine dressed up as tigers and unicorns and fairy princesses as long as I can, while I ponder this.
(So why is a tiger -- a man-eating carnivore -- not objectionable? Harumph.)