I think most people agree the joke was low brow, but don't you think this sets a bit of a dangerous precedent or example if we start itemizing what kind of comedy is or isn't off limits? That's kind of the role comedians serve in life... they push limits, make us question boundaries in the form of expression. If joking about an auto-immune disease is off-limits (which, honestly, is really not as bad as other topics), how many lines do we have to draw for what is or isn't acceptable and what does or does not constitute a violent response?
If alopecia is off-limits... then we should probably add a few thousand other things to the off-limits category.
I agree though that it was probably one of the more tame things to joke about regarding that family. I mean, the low-hanging fruit would've been Jada's infidelity. But even then, I don't think the joke would've warranted violence.