It was definitely mentioned up-thread the last time this came up, but people said the same thing about a black Heimdall, and Idris Elba knocked that role out of the park. Can we at least give them a chance to see if they can be a good more-than-skin-deep adaptation of the characters before getting upset?
Heimdall was sort of an ambiguous figure in the comics from what I read, like he didn't have a normal form like the other Asgardians, he was just sort of a dark figure, which I actually thought was clever and creative to cast as a black actor (and frankly having someone else in blackface, even if CGI'd and not with makeup, would have really been awkward). Plus Hogun of the Warriors Three is of the Asian persuasion so that makes it difficult not to ask, "Okay, so if there IS diversity in Asgard after all, what's the sense in having just one minority face in a sea of whiteness?"
But with Mordo, like I said, Eastern European history doesn't exactly scream 'our royalty/nobility here is black' so that's just eyebrow-crinklingly bizarre. Plus if you replace, say, Chow Yun Fat (who would probably be perfect for the Ancient One) with Tilda Swinton, then not only are accusations of whitewashing (as my understanding, it's defined as replacing minority actors with white ones in a deliberate effort to minimize exposure to minority actors - and I repeat, I am against this in either direction so this is not rooted in racism) in Hollywood going to be pretty well justified (and justifiably rebuked), but then we also lose that whole Karate Kid dynamic Ralph Macchio and Pat Morita had, which is so iconic.