I agree with Lani on this one. When I was a young lad, probably about 5 or 6 there was a poor family that lived in a run down house close by. The daughter, who was overweight for her age was the constant target for ridicule and taunting. Years later, when I had matured enough to think about such things, I began to think about what we had done to that poor little girl. Why had we made here young life such a living hell for things that were simply beyond her control? The guilt of this troubles me to this day, but I have learned from it and have never ridiculed anybody about their physical shortcomings to this day.
Picking on somebody's mental or physical defects is simply and easy "go to" style of insult for some people. I don't like it when professional comedians use this tactic, and I certainly don't like it when friends or acquaintances use it either. It simply shows a lack of thought, intelligence, and wit in my opinion.