Now, if it was a movie that was actually trying to depict Norse mythology, rather than a comic book movie borrowing elements from Norse mythology, I would have argued that it is an odd casting choice. But it's a comic book movie, so it doesn't matter, since it's already guaranteed to have next to nothing to do with the original mythology.
They could've casted a goat in a fez as Heimdall and I still wouldn't care, because it's still a comic book movie.
I will note that it seems kind of ironic to have a black man portray Heimdall, who was referred to as "the white aesir" in Snorre's Edda, though.