Wii Music seems to have a different effect on just about every person who picks it up. The game definitely requires the right atmosphere, and the right people to enjoy properly. Most importantly, the game requires you to play with others in order to truly experience its magic.
To say Wii Music is aimed and can only be enjoyed by children would be absurdly wrong, as would saying that Wii Music can only appeal to those who have no musical experience. I'm pretty sure there is no definitive metric to determine its appeal.
If people want to argue that the game is probably not the best of titles to carry Nintendo through the holidays, I would agree. The game has gotten an unjustifiably bad rap, and as such, it has become a hard sell with the "hardcore" crowd, and is likely not being promoted in stores staffed by such gamers. Nintendo probably should've opted to not allow hands on time with the game on the E3 show floor as it gave many journalists a bad impression of the game due to being featured in an improper setting.