Context matters here, as well as intent. I think there's a distinct difference between a sketch like this (or say, RDJ in Tropic Thunder, though they're different for obvious reasons) and someone in "real" black face, with the purpose of making a caricature of a black person to be simple minded and further negative stereotypes. It is clear, though, that Kimmel has changed a LOT since this was filmed. He's a different person, and I'm sure he isn't proud of this.
The sketch is in terribly poor taste, and it's offensive and stupid, but we shouldn't conflate it with the actions of genuinely racist and abhorrent behavior as a form of "whataboutism".
Also I don't think he's defending Roseanne or her actions, he's just saying she's got some other issues and asking people to not attack her (not saying not to criticize her, but specifically attack). I don't think we should be going after Kimmel for any of this, seems pretty absurd.
Edit: Just to be clear, I understand the difference between what TT was doing (mocking people who don't understand blackface and painting the image of his character being a total piece of shit) and what Kimmel did on this sketch (which was just poor taste acting, and has grounds for outrage for sure). I just don't think Kimmel or the writers (or anyone else who was involved) meant to demean black humans with the sketch.
