Hmm...I think that Meg has been dissed far more often than Chris has. Chris has had some moments of genuine respect amongst his family and friends.
As for Joe, it's not that he gets treated like garbage, it's just that FG's writers are constantly writing gags and jokes where the audience is invited to laugh at his affliction. They keep writing more bits that draw attention to Joe's disability and make it the subject of laughter. It's not so much that the character receives shoddy treatment, it's just another example of the "How low can we go?" and the "How much can we get away with?" stance that the FG writers like to take.