i'd have to agree and say a mario paint game would sell better than bob ross anyday, only because non-gamers and gamers alike will pick it up, with gamers picking it up quicker. i see what you're saying Spak but in order for that to happen realistically is if the non-gamers buy in to the wii at anyway.
someone who bought wii with wiiSports would later pick up a bob ross game for all the points you made, but more realistically there are too many gamers out there that have played mario paint and have been waiting a sequel (especially those of us that didnt get to play the 64 game) who would drool over it.
i feel as if the two games could only coexist if they were drastically different: if mario paint went the route it took with the 64DD games and a bob ross game stayed true to what bob ross does (did)
bob ross only painted landscapes (with happy trees!), not portraits or still lifes (atleast not to my knowledge...) so having him do that wouldnt be bob ross at all. i'd expect there to be two styles of play. one would be just like watching the show, in which bob would be painting a landscape and you as the viewer at home could follow along using his tips and recreate his painting. you could be scored for accuracy and maybe like brain age, show you how much you've progresed over time as you get used to using the Wiimote as a virtual brush. such a mode could be played, like someone else said, in two screens; one with bob ross and one where your canvas is, or even have a picture-in-picture type screen where a thumbnail of bob ross painting would be, getting close ups on the parts of the painting he's doing to help you get more detail into yours.
with mario paint, its established as a painting/composing/animating software so it wouldnt feel out of place to do those sort of things, hence a bigger game. bob ross would have to be a "touch generations" type title that would be $20-$30 because there isn't as much you could do. online features would be nice, like the art gallery on wiiCulture mentioned earlier. you could even 'auction' off some of your works, that coudl apeal to the non gamer crowd as well as gamers.
mario paint already has an established market though and it would be hard to topple if the two games co-existed at the same time, especially if both were devoloped by nintendo. leave the bob ross game for someone else, then down the line release a mario paint game. its the only way to go (IMO)
oh and dont get me started on a drum game ;-)