What can I say. I just remember a time when solid humour was based on wit rather than pointing at people who look a little different from the norm and thinking they are fair game for a giggle. This wasn't a standup gig where you risk being lampooned, it was an awards ceremony.
I'm not really making a big deal out of it, I'm genuinely saying I'm not sure how I'd have reacted and I can't rule out that I wouldn't have taken the stage. If she was bald due to cancer treatment I would have to have been physically restrained. If I was bald due to cancer treatment and the joke was at me, I'd probably have tried to get the microphone, explain why I was incensed and denounced the swine. If it had been a derogatory remark about hairstyle or clothing about me or my partner I'd have remained seated and just considered the comic to be poor. But for this exact instance of this example I have no idea what I'd have done no matter how long I think of it.
My point being, I'm not without sympathy for Smith based solely on the circumstances of that 15 second exchange in isolation as I don't see how holding a microphone and standing on a stage should impact on whether something you say is acceptable or not (or on the response you should receive for opening your mouth). So I've no issue with him slapping the guy on live TV, offering a belated apology and nothing else happening.
In terms of disfigurement, Google "alopecia' AND 'disfigurement' and you'll see loads of entries associating that term with the condition. Whether that is a medically accurate term I don't know. I also don't know anything about the woman in question, but I think its pretty fair to assume you should be twigging that a bald woman may have a medical issue with greater mental impact than male pattern baldness. Ergo, probably not a suitable topic for humour in a public setting and if you elect to go down that route (particularly if premeditated) then you should realise there may be consequences.
Just because the woman isn't choosing to wear a wig or undergo surgery to get hair implants wouldn't change my opinion on whether it is a fair topic for humour. I'm struggling to think of a comparable Male specific ailment that is physically visible to challenge if I'm being a bit sexist here, but I don't think I am.