A portion of a Rolling Stone interview with Miyamoto...

RS: There are a lot of fantastically strange instruments in the game, like the cat suit. What was the inspiration behind those?

Miyamoto: The typical pattern when we're developing our games is that the development teams will work very diligently to make the game and I'll take a look at it and say "why does this game feel so serious? You need some more silly things in there." But this time around I didn't actually have to do that because the team was creating all of those strange instruments of their own accord. Even the cheerleader, which is another instrument in Wii Music, is something that I didn't even have to suggest to them; I just turned the game on one day and all of a sudden there was a cheerleader. From that sense I would say that Wii Music was a relatively easy development project for me.