I don't agree.
I don't see why you say it looks better up-scaled on Dolphin.
The Wii U remake is using updated shaders and new textures to enhance the look of the game and make it brighter and bring out the color. It's also a safe bet that they're updating some of the character models and animations to make it appear more modern.
Also... gameplay enhancements. Touch pad control. High-speed sailing.
Aside from all that, let's just talk about presentation. The remake runs in native 1080p, and it's in 16:9.
Even if you upscaled the original to run in full HD, it would still never be 16:9 unless you stretched the aspect ratio, but then it distorts the images and makes it look terrible.