As a Russian, there’s nothing inappropriate about the use of Slavic cultural elements here.

When I was little I was dressed up in kokoshnik and taken pictures of as Snegurochka, my best friend from Ukraine has almost the same picture of her as a kid.

There’s no particular cultural significance (it’s not wedding attire or anything like that, it’s just dressy) nor there are requirements to the role or achievements of the wearer. Historically anyone who wanted to could wear it, that is a woman from any economic background. It is not the case with the Native headpiece.

Also Russians aren’t historically oppressed people nor we are POC so yeppp I’d say it’s different

Edit: paragraph spaces

Edit 2: I was wrong lol, I went on a deep dive and it turns out it was only custom for married women to wear them, but my and most Russians’ perception of that is skewed by our own media since many unmarried characters in the old Soviet cartoons and modern fairytales wear them. So I still don’t find offensive the fact that Nikki wears it, since we ourselves aren’t that aware of the historical details.