This is a question and i'm not trying to dunk on the tweet
I thought the main reason why those were bad because it mocked black people and they were the target of the ridicule, and while Cohen uses a similar technique in Borat he is targeting Americans as part of his satire, not Kazakhs.
I saw a criticism in vein of "People he can mock by making them think Kazakhs are actually like this would also believe that there is a country called Loftristan and its people act like that" and i can accept that, but comparison to blackface makes almost zero sense to me.
I like the movies, forewarning that I am just a white American, but to me the objects of ridicule are the Americans around him. The viewers that may use his character to espouse opinions of Kazakhstan are people who were already hopelessly racist to begin with. To me it's like trying to blame violent video games for violent kids - the kids who act out tend to actually just have the predisposition for it regardless of the video games they play. Not to say it isn't worth avoiding validating those viewers, but I think they're in the minority. As far as I understand it, it's a Jewish man mocking anti-Semitic culture, which doesn't bother me much. My partner is the opposite and is one of the people who doesn't think being racist as a joke is ever funny, which is reasonable too.