I think the show does go over the top at times and certainly is not for everyone, but the problem is that those who are not offended by the language either use it all the time themselves or tolerate it but don't use it themselves.
The show is often quite good but the movie is about an hour too long.
I think there's also something of a strategy by making Mr. Garrison the homosexual (bisexual?) teacher be an extreme conservative on every other issue. While this is technically a possibility, I think it's also somewhat of a means of making conservatives more accepting of homosexual conduct.
But that's about the only left-leaning tendency the show has in my opinion. Yeah, they make fun of Jesus but that's not necesarily conservative, especially if you put it in the context that they are really ridiculing televangelists who are often an embarrassment to Christianity.