yeah, depends a lot on mentality of the person in question. it's generally easier as a matter of principle/practice to just not be rude at all, because then you don't have to figure out which one upsets someone less.
i think you are right that some people would be bothered more for being mocked for being fat or wearing ridiculous clothing as opposed to a skin condition, hair loss or height, though for some the other way around is true. i guess it comes down to whether one is comfortable with/willing to own the things they can control, and how much the things they can't bothers them.
interestingly, i bet that even in your example with the kid, different kids will be bothered more by one vs the other. some kids do not care about drawing, to the point that being called bad at it doesn't mean any more to them than being bad at "playing house" which the kid doesn't want to do...but might take their actual house being called ugly more personally because it's special to them
i don't know much about jada, so can't say which hypothetical hurts more. nor whether one would have upset will more than the other.
Its clear in the video of the event that she is bothered by the joke. Her facial expressions and body language all support the notion that she was upset or at least very annoyed by it. When people are being roasted, particularly famous people that are used to being on camera, they will often genuinely laugh at themselves, smile at the jokes, maybe politely pretend-laugh/smile, or even sort of grit their teeth and give an obviously fake smile but show that they are trying to be a good sport even if the joke annoys them or gets under their skin. For Jada to react the way she did, I think it must have really bothered her.
FWIW, I have a different take on Will's initial laugh at the joke about Jada. I've read that Rock basically went off script with the shot at Jada, but what setup the joke was Rock joking that Javier Bardem would not want to win the Oscar if Penelope Cruz (his wife) did not also win, so he would be praying for Will Smith to win instead, the implication being so as not to make his wife jealous/upset that he won but not her. Then, having directed his attention to Will Smith, he took a shot at Jada. My hot take is that since Jada is also very famous, Rock did not want to ignore her, having just acknowledged both Bardem and Cruz.
I think that might be why he went off script in the first place and made such a hamfisted joke. I think he felt like he had to acknowledge Jada and he just said the first thing he could come up with since he had no prepared material. He even began his dig at her by pointing and saying "Jada, I love ya..." You could almost feel the
"Oh crap there's Jada Pinkett-Smith! I've gotta acknowledge her too!" The fact that in response to some jeers from the crowd he immediately said "Aw that was a nice one c'mon" or something like that, indicates to me that he felt that the joke was a nearly innocuous throwaway line, that was simply intended to acknowledge Jada's presence since he just mentioned her husband.
In any case, going back to the original Joke about Cruz and Bardem, and the reference to Will Smith... Smith was laughing at that joke and I think that his laugh at the quick quip at Jada was an extension of that. I'm speculating here of course, but I think that Smith's laughter at the first joke was more polite laughter than a reflection of him thinking the joke was hilariously funny and I think that spilled over into the Joke about Jada. Will Smith may have been laughing, because that's what its polite to do when you are in the front row at the Oscars, and then when Rock made the joke about Jada, Smith just continued with the polite laughter, until the actual content of the joke hit him... like "Ha ha ha ha... wait a minute... WTF did you just say?" Then he looked at his wife, saw she was upset about the joke, then felt embarrassed for having laughed at it, and felt the urge to quickly do something to demonstrate to her that he was not OK with the joke.
All total speculation on my part of course, but again, that's my hot take.
Not an average one at least. Now if the doggy was handicapped, or dead, that's different of course. As an aside... I lol'ed at the thought of some guy running past you while cradling his dog in his hands... must have been quite a spectacle... and so fun for the dog! 
