I guess a reasonable line to draw is humor mocking someone's medical condition beyond their control.
If we were at a bar making fun of your kid in wheelchair with MS or with childhood leukemia, is that fair game? It's just a joke, right?
The 1st amendment doesn't limit Poor taste, or a Bad sense of humor. Although Chris Rock may have hurt somebody feelings, he never physically touched anybody. And I do not think it is the same thing as mocking somebody with an obvious disability, or a child. I agree that these were adults, that seek the public limelight and IMHO as with politicians they are fair game. After all its an award show, that says "look at me look at me".
In the Video Smith is seen laughing at the joke although his wife did not. I wasn't aware of her hair condition, I guess I forgot to renew my subscription to Variety & the Hollywood Reporter. Although ignorance may not be a good enough excuse, neither is battery. The travesty is the Lack of, or Equal application of law.