It didn't seem so smooth to me. It had very noticeable frame rate drops in the early areas I've seen. I think the grass on the fields is a major culprit, as running through a grassy hill seems to murder the frame rate. It seems like a great game in terms of design and artistry, but it's not polished the way I'd expect a top Nintendo title to be.
Looking at the DF comparison it looks like Wii U suffers way more than Switch. So often the Wii U will drop to 20FPS 3 or 4 times as often as the Switch will.
^ digital foundry video. Oddly, the WiiU locks at 20 fps at "better" locations, imo. Like heavy foliage instead of fighting.
It's really hard to tell what's causing the frame-drop for either of them. It almost seems random. I don't notice it on my Wii U but I'm not a console/TV gamer so I can't compare it to other console games. My PC monitor is about 18 inches from my face and the TV I was using in maybe 15 feet away.
So yeah, undocked its very smooth, but on the TV it is choppy, but after playing for a few hours I didn't care. Lovely game