A lot of 3DS games have pretty decent models, which make the move to platforms with higher output resolutions paired with higher quality textures. Look at the Ace Attorney games and their mobile ports.
The 3DS Pokemon models are already high quality, and you can look at them in any model viewer to see that.
They'll likely use them with high-resolution textures and bump maps, along with a good lighting engine.
Even without those things, the 3DS games look pretty nice upscaled in Citra, so I have no doubt the games will look great on the Switch.
