Colbert is a character or concept comedian (something that wears out fast with me, like Bobcat Goldthwait's creepy whining). His concept is that he's a big lib playing a lib's cliche idea of an arrogant, boneheaded conservative blowhard. It's a one-note idea based on a false stereotype. Therefore, what might appear to be self-deprecating humor that makes the comic look ridiculous is actually an attack on people he doesn't like, so it comes across as nasty rather than funny. That's why liberals like it so much: it reinforces their own prejudices and allows them to smugly belittle everyone who might disagree with them. But it's bad humor. Good topical humor is not blindly partisan: it skewers hypocrisy wherever it is found. Otherwise, it's just mean-spirited propaganda disguised as humor (see the smug sarcasm of Al Franken, which has the sentence structure of jokes, but seldom displays any wit or even linguistic creativity).
That's a succinct and well-written analysis of Colbert's humor. Good job and thanks...it's a nice change to read an intelligent thought process as it develops.