I find it hard to get too worked up over this. As someone else mentioned here, he wasn't making fun of the mentally handicapped, he was making fun of himself. Granted, he did so in an inconsiderate way, but I really don't think there was any malicious intent here. Maybe that's just my politics showing through.
An aside - when I was in high school, we had a few mentally challenged folks in my gym class. One time, our class went bowling. One of said mentally challenged kids was on my lane. He kicked my ass. Solidly.
As someone else said, I am not sure why people are in a uproar over it. He was obviously goofing on himself and wasn't making fun of the disadvantaged.
As a nation, we are so overly sensitive over things that aren't that big a deal. As far as I can tell, no special interest groups for handicapped citizens have not asked for the president to be impeached so I would just let the sleeping dog lie.
Chuck Norris drives an ice cream truck covered in human skulls.