But “Mumbo Jumbo,” which has been viewed almost a half-million times on YouTube, can hold its own artistically. It’s a mini-horror film, with Whack rapping through a bloody mouth as she leans back in the dentist’s chair, then emerges onto a postapocalyptic streetscape where she’s horrified to see that the zombielike characters that surround her all have the same creepy, over-exaggerated smile she does.