Idris Elba as Heimdall

It didn't really bother me, considering Elba played the role well.
However, the reason people gave to justify it seems a bit flimsy. They claim that the Asgardians are an alien race that came into contact with Scandinavians centuries ago and that the advanced technology the Asgardians possessed seemed like Godly powers to the humans. The humans then began worshipping them and told stories about them.
Being white, they assumed the Heimdall they heard about from some other Asgardian would look like they do, as the Asgardians they had met did.

So having an Asgardian who resembles a person of African descent is purely coincidental. We see him as black, and Thor and the others as white, but that's just parallel evolution or something.

People make similar arguments for why the Saiyans in Dragon Ball Z look like humans, and people argue further whether they look white or asian.

I'm not great at math, so I have no idea how to figure out how unlikely it would be for an alien race to have evolved to look practically indistinguishable from humans is, but my gut tells me that many factors would make it unlikely.

Unless the concept of an alien ancestor from whom all humanoids on various planets all descended from is the reason.

Why the hell should race matter, when it comes to playing aliens? Seriously, why would it matter if an alien has pink skin or brown, because they're not a human and human racial divisions mean nothing on their world. They could as easily have lime green skin as brown, or no skin at all, because they're AN ALIEN.

When it comes to playing Asgardians in movies, what matters isn't skin colors, it's gravitas and/or cool. Elba has both, so he's well cast.