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).
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.