Edit: Just so non-Witcher fans get it.
A good actor is a good actor, regardless of their race. I appreciate accurately portraying a character, but non-white Ciri wouldn't be it.
Ciri is stated to be white. Her biological parents are stated to be white. Ciri carries the Elder Blood - the genes of elfish sorceress Lara Dorren - within her, and is sought after by several characters throughout the story because of it. Lara Dorren, an Aen Elle elf, was pale-skinned, like other Aen Elle elves, and her descendants take after her. Ciri is the least suitable character for portrayal by non-white actors among the Witcher cast.
Remember [Death Note](https://en.wikipedia.org/wiki/Death_Note_(2017_film)) and [Ghost in the Shell](https://en.wikipedia.org/wiki/Ghost_in_the_Shell_(2017_film))? Beside being nothing special as films, they Americanized established and well-received works of Japanese fiction for no reason. If they made T'Challa white in Black Panther, wouldn't that be upsetting for fans of the setting and character?
Making her black/Asian/etc. to combat modern or in-story prejudice and racism is also pointless, since the story already deals with such matters. It's an incredibly stupid decision that shouldn't be made.