The console is rendering the graphics in 900p and upscale them to 1080p to display them on HDTV's, meaning serious frame drops some times. One quick google search will bring you to that and reviews mention it as well. But again, the hardware is lacking here, not the game (although not developing the game for Wii U in the first place maybe would have circumvented this issue)
For a start, upscaling doesn't quite work the way you think, but that's beside the point.
There aren't "serious" frame drops for one thing, nor does this game have major pop in issues. It also runs identically on portable to screen mode since the patch in my experience, and I'd be the first to moan about framerates. Is it 100% smooth? No. But then is Witcher, Horizon or Skyrim? Or any other open world game? No.
In terms of power, may I ask what you were expecting? Did you think this machine would make a dedicated home console like the PS4? If it did, it would no longer do what it's supposed to, and I for one don't need yet another console like the ps4 of Xbone. This is a mobile gaming platform that you can use on the tv.
As far as Zelda graphics, they are beautiful actually. As I'm sure you know, graphics aren't all engine and power they are as much art style too. As
@Bill.s.preston said earlier, it's very much like a Ghibli film and I love that. You might not, that's fine, but they are pretty far from being terrible.