Sure Norse mythology is a cultural staple for Scandinavian people, but I don't see anyone of these people complaining about the inaccurate representations of zombies, which believe it or not, are religious in origin. Zombies come from the African/Haitian syncretic religion of Voudoun. However, most zombmie movies are about viral outbreaks rather than Voudoun priests. No one complains about how the Native American Wendigo is treated as if it were a werewolf like creature, which is an inaccurate representation of Native American cultural heritage. If it's good for the goose, it is good for the gander. Either no one changes any cultural history when reimagining/reinterpreting it or everyone is free to reinterpret it.
You need to educate yourself before you draw conclusions.
Zombies are in the myths of many cultures even in Chinese culture with Jiang Shi. Korea, Japan also have them. Zombies or undead deal with the paradox of the human spirit (one moment the body is full of life, next it's dead and that one last breath redefines what this mass of muscle and bones is...) and it's a common thematic element in basically ALL cultures, going back to the Egyptian and Babylonian.
As for werewolves and lunar creatures that also is a universal concept.
You need to learn that ignorance isn't the basis for an argument. Because you either choose to focus on a small element of a larger idea, or because you're geniunely ignorant, doesn't give credence to your position.
You're like someone who argues the bass line in Ice Ice Baby is the best ever because you've never heard of David Bowie. Because you only know about the context of voodoo for undead zombies doesn't mean there has NOT been a 5000 year tradition of them all over the world.
Simply put zombies and werewolves are not New World concepts. You sound ridiculous.
As for Heimdall being black it's distasteful because some people value the concept of artistic integrity.
Thor the character was specifically based on Viking Myths. If you're okay that the content of art or cinema is influenced by the fashions of the society politic (in this case the idea that casting a black character insures greater box office from "ignorant blacks" who can't relate to a movie without a black face in it... or are otherwise too stupid or ignorant to appreciate Viking myth without dumbing it down... all very racist Hollywood ideas), then it's fine that the dude is randomly black.
As for why it's a big deal.... well it speaks to the fact that as a society we really really notice black and white. Other racial things are more hidden and sublimated. For example why are so many of the actors in these Marvel movies Jewish (like the entire cast of Iron Man, Jane Foster and Kat in Thor etc) when the characters are not? Who cares? 75 years ago it would have been a huge deal given the political climate then.
So basically the answer to your question about "the fuss" boils down to 2 reasons
1) because it's stupid that Idris Elba is Heimdall. and some people dislike stupid things
2) because white people really really notice black people in anything, and that's the way things are (the way ppl are wired evolutionarily etc)
this is so weird.
