I still play Wii and Wii U games, usually on my Wii U.  While I've played Wii U games on the just the gamepad using the off-tv function, I've never done that with Wii games before because the gamepad can't be used to control Wii games.

But today I played Super Smash Brothers Brawl on just the gamepad, which was kind of awkward but ok because it doesn't require motion controls.  But the visuals on the gamepad shocked me.  I'm used to Wii U games looking worse on the 480P screen than on the TV, but Brawl looked so much better.  The decrease in jaggies was expected, but the textures looked so much better than I remember them looking on my old SDTV in 2008.  It looks like it could have been an HD game on the gamepad.

Has anyone else had a similar experience with Wii games on the gamepad screen?

I see where you're coming from. I have Xenoblade Chronicles on the Wii U, and while I wasn't as shocked as you were, I certainly prefer to play it on the Gamepad over the TV.

The last game I played that way was The Last Story. It was great. I forgot the Wii U could do this, actually. It even has a built in sensor bar.

That's because you are comparing it to an SD TV from 2008. Wii games on Wii U with HDMI on a modern 1080p TV look pretty good (Nintendo games, at least).

i used the gamepad screen a lot to play Wii games. and it really makes me want the Switch to have Wii games available on it for that reason

It’s because the screen is smaller, makes it look sharper.

The only TV I have now is a 1080P TV and the only way I play games on it is on the Wii U with HDMI cables.  Brawl at least is a jaggy mess and the textures look much worse on the 1080P screen.  Games like Mario Galaxy, Prime 3, and Skywards Sword still look great on the TV and I haven;t tried them on the gamepad, but Brawl does not look good upscaled to in 1080P and look much better on the gamepad.

Wii games on the Wii U gamepad look great. Much better than on an HD/UHD TV. If we talk about... 32 inches, it's not that bad. But when we have a 42 inches TV or more... either you play VEEEEEERY far away from your TV or you're gonna see more pixels than in the NES era...