With Kang, his race is irrelevant to the character - he's from a thousand years in the future, and looks like
this 99% of the time. With Valkyrie, that's more of a title than a specific character; as we saw in Thor Ragnarok, there are (or rather, were) many Valkyries.
There's a TV Tropes page that seems to sum this up, called
Ability Over Appearance. (I'm not sure what languages the site's available in) Take the Shawshank Redemption - the character Red was a middle-aged Irishman, but they cast Morgan Freeman for the role, and I don't think it's possible to picture anyone else narrating that movie.
It helps to try and keep an open mind; changing something from the source material doesn't necessarily have to be a bad thing. There are cases where it can backfire, but take Dr. Octopus from Spider-Man 2, for example. He's not much like the comic book character (he's a mentor to Peter, he has (or rather, had) a wife, the tentacles have an AI that's controlling him), but Alfred Molina gave such an amazing performance that he's one of the best-remembered supervillains in movies, even now.
Likewise, the villains in Tim Burton's Batman movies weren't much like the originals (the Joker had a clearly-defined backstory, the Penguin was a deformed man living in the sewer, Catwoman was a secretary motivated by revenge), but I don't think anyone's going to say that they were terrible because of it. (heck, I would have loved to see Billy Dee Williams's interpretation of Two-Face)