I think this is an underappreciated aspect of his work. He occasionally strays from the path (see e.g. "The Night Santa Went Crazy"), and some of the polka medleys do incorporate some pretty dark songs about sex and violence (e.g. Stone Temple Pilots' "Sex Type Thing" or Foster The People's "Pumped Up Kicks") but it's a good rule of thumb that if a parody/comedy song is particularly profane/offensive/mean-spirited, it's not his work.