A good deal of a joke though, is in the delivery. And with two retarded main characters, the delivery is always going to be bad.
Then there's the juxtoposition elements, the exaggeration elements... Shock and/or outrage elements. Subversion elements...
The closest to any of that in the little song at the start of the clip was the sewer mutant grabbing apple and dragging him into the water. Which while a little funny, wasn't enough to warrant the blandness of the song. Hell, even though the whole "It ain't in the sewer" bit is a subversion, it is such a nonsequiter that it is actually anti-funny.