I don't think the joke was tasteless or malicious. It feels like many who are trying everything they can to give Smith some kind of pass are trying to play it off like Rock was making fun of Jada's condition, but, no, the joke was about her lack of hair. Had he said, "Damn, Jada, that alopecia is really giving you that G.I. Jane look," then, yeah, but he said nothing about her disease. The joke was about her hair, and it wasn't even an insult! G.I. Jane was a badass character from what I remember, so being the next G.I. Jane is almost a compliment, even if it was meant to be a joke.
It is also worth noting that Jada is a celeb of sorts, as I have seen some chatter that she should be off limits because she is Will Smith's wife (how sexist is that??). No, she is a celeb of sorts, has her own show where she has been very public about things in her personal live, and apparently has over a million followers on Twitter (which apparently matters). She is very much a public figure, thus fair game. Children and partners of celebs should be off limits, yes, but while she may not have the pedigree of Will Smith, she is still a celeb herself.