Some people complain that there was a black lead at all. Since this kinda had a hint of racism, it seems that some of them have switched to complaining he is the token black guy. Of course, if there were multiple black guys they would complain they had ghettoized it or some such bullshit.
It is all bullcrap anyway. Finn's race has nothing to do with the movie and isn't mentioned or referenced at all. He could have been cast as any color and he would have been fine, but he did a good job in the role he was given. He was great in Attack the Block, which gave him a high profile at the right time, so it isn't like he was picked from nowhere. He auditioned for 7 months for the role before he got it. It is absurd to attack the guy.