She does it to actually get rid of her power and her "destiny" (which is to birth a child that'd become THE BIG SHIT, Sapkowski's own Kwisatz Haderach) coz she's sick of it.
Abandoning her destiny to become an Empress with world altering powers to become a mutated witcher is a act of cowardice not courage.
As said in my opinion her choice is a tragedy which would make her miserable rest of her life.
Yet I can bet that ingame she will be portrayed the complete opposite, as strong independent girlboss, that dont neen no man, that is so badass for her choice and with no regrets for her life choices.