Well this is a derivation of an ever-popular favorite, ass clown. The only difference being the substitution of one profane word for another. Shit clowns are people who you would normally consider ass clowns, but they smell really badly. So the use of this word is usually saved for those ass clowns with poor hygiene, including but not restricted to bums, beggers, democrats, Rasheed Wallace and the like.
n. That shit clown just asked me for what I could spare so I gave him an application for McDonalds.