Well despite what the internet would try to tell you, Smash for Wii U is much more similar to Brawl, both visually and mechanically, than Ultimate, so I guess you could say it was an HD game on the gamepad.
Personally, the only Wii game I played a lot of on the Wii U was Metroid Prime Trilogy, and those are too immersive for the Gamepad. I did try it a few times, and it was a no-no. I think they look fine on the TV at 480p.
Edit: and before anyone says otherwise, here are some videos.