Yes, execution is much more important than the initial idea.

Though the idea is still relevant. Think of it like this. An excellent idea with terrible execution won't be able to get any attention and sales. Imagine everything glitching out, bugs everywhere, looks terrible, runs at 10 fps. No matter how brilliant your idea, that's just not enjoyable.

But certain ideas are harder to execute than others. If you do something entirely new there won't be existing fan bases and communities you can share your work with. Getting a person interested is gonna be much harder. Similarly, if you make a game in a genre no one plays anymore, there's gonna be fewer people looking for a game like that out of their own motivation and it's gonna be harder to get people to even look at your project.

So it's not like the idea is irrelevant. It still has a lot of consequences. It can make your path towards a successful release easier or harder. But if your execution isn't good enough there's not even a chance for a successful release.