Yeah, I feel the same way.

There's always going to be dickheads on the internet, and I think in this case it's a very small group and of that group you could probably count on one had that are actually Dr. Who fans, if there's any at all.

If it were a significant percentage of fans then there might be something to talk about. But this seems another case of people that don't know anything about a show and just seeing a black actor being cast in a role what was previously played by a white actor and making racist hay.

Anyone that knows anything about Dr. Who knows the established lore allows any actor of any race or gender can play the part.