Dressing up as a celebrity has always been a popular Halloween choice for the non-famous layman. There's absolutely nothing weird about your roommate donning a Ke$ha or a Gaga costume. But when someone who's already a celebrity dresses as another celebrity? Well, that's just an ouroboros of fame that confuses the order or things.
This year, there are tons of famous people dressed up as other famous people, including several Mileys and Miley's own turn as late-90s Lil' Kim.