Personally, I think a TV series based on a book is its own thing, it doesn't need to follow the source material to the letter. It is based on something, but it is up to the authors to create their own interpretation of something. I like to give the example of Idris Elba that was considered (maybe still is) for the role of James Bond. I think he's great, and I think he would make Bond his own, bringing something new to the role. I really don't care what his skin color is - he is a really cool actor, and I can just see him as 007. I really believe that people who considered him for the titular role weren'te filling up any quota or anything, they were looking for the best James Bond for their vision of James Bond.
With Ciri - if they found a great actress for the role, that best fits in their vision of the character, it just doesn't matter what the color of their skin is. Not sure this was the case here, though, as - if I understood it correctly - they still didn't find anyone for the role, they just decided that they should be of a certain race - which makes me question the choice a bit. Now, I will give them the benefit of the doubt - perhaps they do have a good narrative reason for that. What worries me, however, is that perhaps the reason for this change is just to cross something off a list, like, you have to have certain things in a modern, major TV show. For example: "We need at least 30% women in a show" is not the way to do things. "Women can be absolutely fantastic characters, so let's make some really good ones", however, is.
The reason I have a problem with this is not because I don't think various people should be represented and find heroes in the entertainment around them - because I think it's really cool to have that. What worries me is that these types of decisions are almost coming down to some automatism which, honestly, disrespects the very races of people that you are trying to include. It also seems to follow just what USA audences believe is inclusion. As someone who is a Slav from Eastern Europe, I can't remember the last time anyone from this region was represented in way other than being an ex-soldier, terrorist or a refugee. In fact, Witcher is one of the few globally popular series that actually celebrate this culture. Please note - I don't really care about any of that - I couldn't care less if the new Witcher show is about Slavs or Hungarians or African-Americans, as long as it's good. But I am noticing a bit of a hypocrisy when it comes to inclusion - it seems certain groups are more entitled to representation these days, just because they live in the USA. Maybe I'm wrong. Again, I certainly like the idea of young people of various races, genders, ethnicities finding their heroes in modern entertainment. I hope you understand the subtle differences I'm talking about.
Again, not saying this is what they did here, I think we need to see the show before deciding. And I am really excited about the show! But I am sceptical of modern TV shows in this regard, because I do think some of them are just crossing things off a list rather than caring about actual people - of any skin color - or caring about the story. But hey, let's watch the show and see how it goes. Let's trust the authors have some great ideas!
P.S. Actually, while we're talking about her, I was hoping Ciri was of the same age like in the game, they seem to be going for someone younger.