Trey Parker including his ex-girlfriend antics in his animated show South Park is both immature and uncalled for. Relationships should remain private and between the two parties. I'm guessing Parker wasn't innocent during the relationship either, but he seems to only see her faults, not his. He is proving his immaturity by doing this.
As noted by most of the musical hits today, the best number one hits are the ones based on a personal experience, especially when there is pain and strife. The South Park character is being told from his point of view so there is nothing wrong with using someone as inspiration. It is his truth after all.
The South Park character Liane Cartman is based off of an ex-girlfriend of co-creator Trey Parker, who was found cheating with multiple men. Most inspiration for scripts, novels and other literature come from personal experience, so to say that it is too personal to be included in the show would be unjustified
South Park is a show that does not seem to have a limit to the amount of controversy they will throw at their audience. No person is too famous to be made fun of on South Park, and no topic is too taboo to be covered. It isn't particularly surprising that one of the creators incorporated his unfaithful ex-girlfriend into the show when compared with the other topics and jokes covered.