Of course her skin color and racial heritage matters. It's part of the character and background. How important and how MUCH it matters, I will leave to the people who know the character well enough.
If they cast a Jamaican to play Gandalf - that wouldn't matter to you? Ok, it matters to me a hell of a lot - because Gandalf has been very well established through the books and a host of artwork.
In that very same way, I would be quite annoyed if they had him wear jeans or a cowboy hat. If they had him speak Klingon - that would be a very big problem for me.
I have to say if such things don't bother you - you must not care much about being faithful to the material.
Which is cool.
I'm the sort of person who thinks you should be faithful to what you're adapting.
I'm also the sort of person who HATES "historical" movies that ignore the actual established history.
If you're going to do something, then it's my very firm opinion that you should be doing it right.
Of course, I assume that people have brains when I say this. Meaning, when I say something matters - it doesn't mean that one race is better or more interesting than another.
If James Bond drove a pink VW stationcar - that would also matter, but it obviously doesn't mean there's anything wrong with pink or stationcars.
It just doesn't match the established character.
A few things to consider here, as some things you said apply, but some don't. First of all, it's a work of fiction. Just as with Game of Thrones or James Bond or Lord of the Rings, books and movies DO differ. A blond haired blue eyed dude is the most recent Bond, Tom Bombadil was never in the LOTR trilogy. Arwen was never a major story in the books. Daenerys doesn't have purple eyes in the series. Catelyn Stark didn't come back as a zombie.
There are reasonable changes that take place from page to screen, and some of them are big. Sapkowski is creative consultant on this, and since it's his world, why should race matter if he gives his blessing?
Let's also use our brain and make reasonable comparisons here. Bond driving a pink VW is not reasonable and would change the tone (unless it's a Peter Sellers or Mike Myers parody). Going for a black Bond (like the internet has been talking about) is indeed reasonable and wouldn't have an effect on the tone. Gandalf talking in a Jamaican accent straight out of Cool Runnings would not be reasonable. A black Gandalf would have backlash i'm sure, but a
Morgan Freeman Gandalf wouldn't be unreasonable.
In regards to Ciri, not to give spoilers, but it would not be difficult to change her historical background to fit within parameters and intents they have for her on the screen.
Also, it's a work of fiction. Changing the material doesn't deny the history, it only re-interprets it. If i'm watching a historical WW2 movie, it's unreasonable to think the Germans will be played by Asians. If i'm playing a historical fiction game like Yakuza, then race is central to the story. But if i'm watching Batman, who cares what race plays heroes or villains, if the interpreted history on screen fits. It's not central to the story.
HOWEVER! And this is a big however. What I DO have a problem (as others have stated) with is when they pick the race first, and quality of the role second. Changing race just for some political statement, instead of picking the best actor for the job is ridiculous. I don't have a problem if Morgan Freeman played Gandalf, but I do have a problem if they picked Morgan Freeman over Ian Mckellen to satisfy some far left agenda, rather than pick the best acting chops for the role.
