Not the art style, I mean graphically that Zelda on the Switch looks better than Witcher 3 on the switch. Tecnically Witcher 3 is leagues above BOTW on other platforms, I'd disagree with the comparison on the switch though.
It is down to art style though, It's easier to make Zelda look better because it's art style is less technically demanding. It's a poor comparison to make when trying to determine if the Witcher 3 port could have been any better than it actually is. How good Zelda looks is irrelevant.