First off, I absolutely loved Mario Paint for my SNES. I played it endlessly and made several creations and animations which I recorded onto a VHS tape (since you could only save one creation at a time). The mouse worked really well and was very easy to draw with.
I don't own a Wii; but even if I did I still wouldn't buy a Wii version of Mario Paint. The reason being is I now programs on my Mac such as Adobe Illustrator, Photoshop, and Flash. Unless, Nintendo came up with a way to make a Wii Mario Paint experience something more than what creative computer software can offer, I'm not sure what the selling point would be.
Unless they included a Wii version of the fly-swatter game...then I'd consider it lol!