70% Player, 15% character, 15% synergy I'd say. My argument as follows: I am OK at this game at best. I'm a Mileena main. Using my main that I know the most matchups with, know the in-depth character nitty gritty with, I am terrible. I don't play well at all.
Then I switch to my secondary, Sonya and suddenly I can play MKX. I'm still far from great at it. I still make terrible decisions, but i'm getting wins in places the character I know the most about (in terms of managing frames, manipulating frames, etc.) couldn't even dream of. Sometimes in LOSING matchups.
I don't think Sonya is carrying me
I don't think Mileena is holding me back
I think Mileena legitimately better fits my play style
but I think that Sonya is legitimately better designed.
So I give a solid 70% Player. An excellent player will excel regardless of character choice, but will still meet some character based limits.
15% Character. A well-designed character is going to naturally do a little better than a character that isn't designed to play the same game. (Sonya vs Mileena in MKX: 50/50 BOYS)
15% Player/Character synergy: An excellent player could use an excellent character that just doesn't click with them and have some terrible results. Case in point: Boneshaper at Evo. Lots of good players were rocking the flavor of the month but just couldn't get the wins despite how dumb BS was, because it didn't click right for them.
You have to be able to some extent click with your character otherwise despite having the tools, it won't matter. Give a rushdown brain a really good zoner and there might not be results. Despite the player knowing how to play the game and the character being good, there's a bit of dissonance that can prevent the win
Just my 2 cents.