Why does the author have a running gag about the school changing its mascot's name, and Melinda coming up with mocking nicknames for teachers?

I think this is a part of Miranda's wry and dark and witty sense of humor. Merryweather’s mascot, due to complaints of lewdness, has just changed from the Trojans to the Devils. This represents situational irony. Miranda notes the change is ironic because they would pick a name symbolizing evil over a name that may or may not be linked to sex.