Jon Stewart, Stephen Colbert, and John Oliver are very funny men. Their humor also has a distinct liberal bias. It’s possible that they don’t think they are biased, because they sometimes criticize Democrats as well as (though not nearly as often as) Republicans. But that doesn’t mean they aren’t biased.
By “liberal bias,” I don’t mean they always support Obama. Most of their humor is directed to two targets: first, government officials who lie, waste money, and/or support policies that kill innocent people; and second, people who don’t trust government.