Likable to readers doesn't mean likable within the story. A lot of Tyrion's humor, and his taste in associates, is too crass for the society in which he operates; it's not just his appearance that makes people shy away from him. He's really rather sneering and insinuating. Because the reader is in Tyrion's head and on his side, it's less obvious, but an awful lot of his funny lines are not actually appropriate things to say to people if you care about being liked. Which he doesn't, because he's decided no one is going to like him no matter how polite he is.