A Dirty Teen Mustache is a mustache that some teenagers thinks looks cool on them, but it doesn't. It's like the kid isn't meant to have a mustache, but he has one anyway. That's called a dirty teen mustache.
bawb: Hey, what's up with the mustache?
John: Oh, right, how's it look :D.
bawb: uh, no. Shave that Dirty Teen Mustache off.