I'm afraid this is exactly what it is.
Geralt of Rivia mutated to be faster and stronger than the fastest humans, who has been wielding swords for at least 80 years, is slower than Ciri. A girl in her 20s who showed talent with a sword.
This is without taking into consideration that Geralt was subjected to further mutations which makes him even faster than the rest of the Witchers, and the fact that Ciri is not even a mutant makes it more ridiculous.
In order to defend the ridiculousness of this decision people argue that Bonhart killed Witchers. If Ciri was able to kill Bonhart, then Geralt would have ripped Bonhart's asshole a new one.
And then I hear Geralt is a vessel and Ciri is the main character.
Give me a fucking break.
Why would you think that Geralt is nerfed? He is still badass and someone you don't want to mess with.
On the other hand, Ciri has some supernatural powers, even some that we don't know about yet, this in itself could give her the edge over Geralt, regardless of experience or skill with the sword.
And yes, Ciri is the main person that the story revolves around, like it or not, things are connected and shes is the moving force behind the story.
The Wild Hunt chasing her, Yenefer looking for her, Geralt looking for her, Emhyr looking for her, looks like a patter right?
So yes, she's important and different people have different preferences, so to them some characters are more important than others.
This is no way diminishes the importance of Geralt, he is the one through whom we advance the story and whose choices have the power to change its course.