Yes, Gamecube (and Wii) games are upscaled right up to 4K and they look great. Many games work in 16:9 widescreen also (Gamecube was originally 4:3).
For many games there are even community high-def texture packs that you can add to make them look even better.
[Edit: Sorry, was talking about Dolphin here, not Wii U!]