Henry Cavill is British. He's also not Polish, though, which should be equally devastating to Poland as Ciri being played by a non-white actress.
I think that for the people who actually care about ethnicity in relationship to the character and fiction, context should be important.
If the point is that Ciri should be polish because of some national pride for an important piece of Polish literature, I can't say that's a sentiment I would approve of, but I can respect it.
But if we look at ethnicities in context, then we need to look at the source material.
Jack Ryan may be explicitely white in the books, but making Jack Ryan black would be a complete non-issue. He's still an american man. Sure a part of his background will change, but any resistance on the matter would be rightfully looked at as suspicious.
Making James Bond black is, again, perfectly possible. But as a british man, some extra context and care put in the character's backgroud may be warranted (or not, as GB is one of the european nations with the strongest traditions in diversity).
If we were to adapt the Brothers Karamazov and we decided to make Alexei black, then critics would have a lot more ground to protest - the character is fictional but he's placed in an historical and geographical context where him having a different ethnicity than white would, at least, feel forced.
But the Witcher is a work of fantasy fiction. There is no historical accuracy to respect and more importantly, even if the novels do assign ethnicities to the characters, being a fantasy world you can alter those freely. In a fantasy context, asian people don't need to come from the east, black people don't need to come from the south and the quasi-HRE equivalent doesn't necessarily have to be made by proto-caucasians. As long as Ciri's ethnicity makes sense in the fiction (ie, it's aligned with her father's and the country her father is the ruler of), then she can be anything. If they want to have her be played by a black actress, they just have to make the rest of the characters match. You can have Nilfgaard be predominantly black, if you want. It's fantasy. You can have black people as the guys who wear heavy armors, ride horse and use artillery if you want. That's one of the beauties of it, you have zero reasons to make the fantasy fiction respect real-history or "tropes".
And if you need evidence that "black people as knights" can be rad, I give you Terry Crews in full plate: