Well, why wouldn’t he be?
Is there any reason why Ron shouldn’t be the best at wizard’s chess?
Is Ron brain-damaged in a way that makes it impossible for him to play chess?
When JKR tells us that Ron is the best at wizard’s chess, she’s telling us that he’s the best at wizard’s chess; it’s a part of his character, as much as his hair is red or he’s the tallest of the three.
She gives him a wizard’s chess as a way to show us that Ron is the best at assessing risks. It’s the missing piece of the puzzle the Golden Trio is: Harry charges ahead without thinking of the consequences, and Hermione tries her hardest to never risk anything. We need a middle-ground, therefore she makes Ron determine what they can afford to lose and what they can’t.
Example: in Deathly Hallows, Ron urges Harry to leave Malfoy and Goyle behind in the flaming Room of Requirements, not necessarily because he hates them, but because Ron already has Hermione on his broom and will not risk her death for the other two’s sake. But Harry doesn’t listen to Ron, which causes him to dive back into the fire to save Goyle since he knows Harry will attempt to save both Malfoy and Goyle - better split the load instead of risking Harry stupidly dying as he looks for two people.
Anyway, back to chess -
Chess is also something you learn. But it’s also a game.
To be good at chess, you have to be willing to learn it.
I could very much see Hermione attempting to play chess, not being very good at it, and claiming then that it’s “just a stupid game” and going back to her books because she cannot handle not being the best at something (Divination anyone?).
Chess also involves knowledge of your opponent and predicting someone else’s moves. Hermione is notoriously bad at putting herself in another’s shoes. She is also very by-the-book. There are billions of moves in chess, but only one leads to victory. Hermione’s creativity is rather limited; it’s often Ron who comes up with unexpectedly simple solutions that work wonders (drink Felix Felicis to be lucky enough to get the memory! you and your opponent are unable to do magic - punch him in the face! the sword imbued with Basilisk venom is gone - we have a whole dead Basilisk inside the school!)
Just because Hermione has good grades or a supposedly analytical mind doesn’t mean she’d be good at chess. Just like someone who is very clever won’t necessarily have the best grades in school.
It makes sense for Ron to be good at chess because it’s part of his character. Just like it makes sense for Hermione to read books because it’s in her character too.