"Even Lisa is mocked for being overly-earnest and PC at times."
Yes, because she is nieve and cares so much, not because she is stupid, like every other main character on the show.
Who's the evil guy in town? The rich Mr. Burns. Who sucks up to the evil guy when it's election time in Springfield and makes deals with this devil? The Arnold, the Rush character and other Republicans.
I could go on and on. I'm not saying they don't poke at the left sometimes, but the show has become, in construct, quite liberal.
I really do love the Birch Barlow character.
Also, they do make fun of the Kennedys but that is about the end of it.