I've also heard Shearer's show, and he can indeed be wickedly funny. He can also be strident, a failing that Limbaugh shares. (The strident parts of Limbaugh's show are unlistenable, no matter whether I think he's right or wrong.)
George Carlin is undeniably funny, partly because he's not a liberal. He's certainly not a Republican, but he's got zero patience for the politically correct tropes and feelings-based policies of American liberalism. When he goes sacred-cow tipping, both sides have reason to tremble.
A corallary to your theory might be this: since political humor is typically aimed at the other side of the aisle, conservatives are funny because liberals make good targets -- racing to be called metrosexuals and be "black" white presidents, for example. Conservatives, being so heartlessly evil, make better targets for the outrage and tantrums of liberals -- like when they try to put arsenic in school lunches. Or was it mercury? I forget.
I'm the wrong one to analyze it, of course, since it is precisely the most heartless parts of conservatism that I support.