He was poking fun at himself, that's all.
I think we would all have to admit that we have compared ourselves to a "retard" at least once in our lives. Sucking at being funny is not offensive to me, it's just not funny.
Now if he would have said something like "When I bowl I look like that Corky kid from Life Goes On trying to fuck a watermelon.", that would have been funny. Maybe he needs lessons if he is going to do late night, ooooh I can't wait for him to do Conan...FUCKING PATHETIC.
And no arguing about this. Because we all know,