This short dialogue is from the movie Nick and Norah's Infinite Playlist:
Norah: He's a complete mo, look at him.
Caroline: He is not a mo. Look at how shitty he's dressed. His Supercuts haircut.
Considering this usage might be a slang, I checked Urban Dictionary but couldn't find anything that fits the context. I can only think of mo being short for modus operandi, but that is usually written as "M.O." I think.