I realise this was a little while ago now, but I love that Katy Perry went to some kid's birthday party and made their day with the surprise. You can see in the video for Birthday that these kids thought it was the best thing ever. What is less obviously fun is the (deliberately) terrible face painting she did before taking off her disguise. It's also not clear from the video whether she unmasked herself at the other four birthday parties she attended as various terrible entertainers. If she didn't, then the joke isn't "I pretended to be a terrible party entertainer but I'm really Katy Perry". It's more like "I was an actual terrible party entertainer and then I left", which is less of a joke and more a way to ruin a party for no reason. The only way to "pretend" to be a terrible entertainer is to be a terrible entertainer.
I sincerely hope that, after some awful "entertaining", she took off her disguise at every party and performed as herself for everyone. It might not fully make up for some parts, such as the staged car crash (at one of the parties) that upset several kids and parents, but "Katy Perry staged a car crash at my birthday party" is a more positive story than "Some horrible clown ruined my party and there was a car crash, too". Pranks rarely have positive outcomes for the recipient. It's important to keep that in mind when planning one, because only the prankster and anyone in on the joke end up laughing.
Mokalus of Borg
PS - That said, when Katy Perry gets a real movie role, I'm going to see it.
PPS - Besides "Part of Me", that is. And those one-episode roles in How I Met Your Mother and Raising Hope. And her voice work in The Smurfs.