I suffer with Alopecia Areata.
Personally I didn’t think the joke was super offensive. At least I wouldn’t have been offended by it. If I end up having to shave my head from this shitty disease, I could think of worse things than being compared to GI Jane (strong, positive female? Fine by me)
Will Smith was bang out of line.
And as others have said, he was laughing at the joke himself until he saw Jada’s ‘face’.
As for Jada, I used to like her. I haven’t been keen on her for a while now however. She seems to have no qualms about humiliating her husband.
She also said in a video she made that she wasn’t bothered about people’s comments about her hair. Clearly she’s changed her mind 
Will could’ve reacted any other way but the way he did. This could’ve even been used for a moment of education around alopecia. But instead it overshadowed the entire night.
Ludicrous reaction. And completely out of character for Will Smith. Which makes me wonder if he’s suffering with some mental health issues to be honest.