Last night on Verdict, Abrams was showing clips of Senator McCain's hosting gig on SNL (he's not a celebrity!). Everyone on the panel seemed to agree that Senator McCain is a funny guy. In one clip, McCain is singing random songs for some reason. Cut back to the panel and everyone is chuckling.
I would argue that Senator McCain's performance, or any of his other non-SNL attempts at comedy, aren't funny. The reason barbecue people tend to chuckle at McCain doing comedy is because it's incongruous -- an old United States senator singing and dancing and being silly. How odd! How bizarre! McCain in a fake beard pretending to stalk Amy Poehler! It's like the first time you heard your parents use profanity. It's the difference between laughing at and laughing with someone.
Put another way, Senator Leahy appeared in a cameo in The Dark Knight, which was cool and fun. But I don't think anyone would suggest that his performance was especially awesome when compared to, say, any other actor in the movie. It was passable... considering he's a senator.