I like the Mario Paint music composer because its simple.
Each icon makes its own noise and you paste it on the music bar. It makes it both easy and visual. For those who have no creative abilities they could play around with the (what would be) many premade tunes.
This contrasts to noteworthy composer which is a PC music construction program. Its quite difficult to use.
I guess people could plug in a USB mouse or tablet to make painting easy and they could link it with the miis and photo channel for further add ons.
I think the real value in a paint program will be use across other games - such as shirt designs in Animal Crossing, or sign posts in Mario Kart. Games can have custom sections that the paint part of the program can access.
I would have music, paint (incl colouring/stamps/photo edit), games, and possibly a basic 3D polygon maker where you can make accessories for miis or even a 3D drawn to life.
Mario Kart Wii
02/11/2009 / Diff room to my wii. I can't wii and chat at same time.