Fair enough. I don't disagree that the comparison isn't 1:1, but the sentiment should be just as strong, in my opinion. You could (and I do) look at "cultural appropriation" as a sort of analogue to "objectification", in that you are reducing someone's culture to the aesthetic level for the purpose of accommodating your own desires. Essentially, in this case, the idea of a skinwalker is being "objectified", i.e., reduced to nothing but entertainment, for the good of the writer and reader.
I think what I wrote above is hopefully more clear? It's a sort of rephrasing of what I said in my first post. It's harmful because it misrepresents what they actually believe. What they actually believe is something very few people know about, because MOST people learn about Native American culture (especially tribe-specific culture!) from fiction that is almost never correct.
I think it would be wholly different if the Navajo religion was common knowledge.
For an example of cultural appropriation that I don't have a problem with: see Islamic stuff. I can understand why Muslims do not like it, and I empathize with that, but their religion is not one that no one knows about. The biggest thing they have to fight against is the assumption that all Muslims are terrorists, which is a very real problematic assumption, and one I work to correct at every opportunity. But it has nothing to do with, for example, a recent anime soundtrack having to remove a song because it has Islamic chanting in it.
I do not believe cultural appropriation is a black and white issue.
I do, however, fully believe that this specific issue of misrepresenting Navajo beliefs is one worth considering.
I understand where you're coming from, but, in my not so humble opinion, it's possible to borrow from other cultures while also being respectful of those cultures. In this specific case, I would suggest refraining from implying that the Navajo are wrong to vilify skinwalkers. I would choose another path to explanation. Maybe say skinwalkers are evil creatures, but some Navajo wizards devoted their lives to hunting them down, or something. I dunno, I'm not the writer here, but, like I said above, cultural appropriation is not a black and white issue!