Yes a mistake that should be punished so that he, and the wider audience, understand why it isn't alright to make such jokes. Stop trying to make this a conspiracy. He didn't 'accidentally' do anything, he wilfully made a racially insensitive comparison between his friend and a picture that is reminiscent of the sorts of cartoons that were historically made to demean black people. He is probably ignorant to the cultural significance of such a cartoon, and so will be reprimanded and hopefully educated on such matters. So no he is probably not a racist in the classic sense of the word, but it was a racist, or at the very least, racially insensitive 'joke'. Don't make him out to be a victim just because he plays for City. If you have the leading anti-discrimination body in football (Kick It Out) condemning the tweet, maybe just listen to them?