Graphics on Wii U titles blow Wii's out of the water. The Wii capped at 480p, while the vast majority of Wii U games sit at 720 minimum, up to 1080p. The system itself is also stronger, allowing for higher end shaders, anti-aliasing, and all that other good stuff that I don't understand but know is stronger under the hood.
Wii games are also "upscaled" to their highest graphical limit, but don't expect to see them turn into HD just because you're playing them on the U.
Just don't expect PS4/Xbone graphics from it. The main pull of the Wii U is Nintendo's First-Party in HD along with some indies to keep them company. Graphics and power are good, but not the key point.
Like, a lot of stuff, but also nothing because indecisive