Cartman's mother, Liane, is based on co-creator Trey Parker's college fiancee, who cheated on him before dumping him. Understandably upset, Parker first channeled his heartbreak into Cannibal! The Musical, a film that, according to the audio commentary, he wrote just so he could ridicule her. In it, Liane is a horse that everyone rides, and if you don't understand the symbolism you should probably get back to your coloring book. He also ridiculed her decision to dump someone now worth millions on the commentary, because when you're busying making movies and TV shows you don't have time for therapy.
As for South Park, Liane has a single defining character trait, and it's that she's easier than a kindergarten math test. She's done everything with everyone, starred in scat porn, worked as a prostitute, and was featured in Crack Whore Magazine, amongst many other sexual shenanigans. So we guess the lesson here is: Never screw around on a guy that's good with paper cutouts. It'll come back to bite you on the ass in a big way.
Comedy Central Productions
Literally and figuratively.