The main reason I wasn't interested in Wii Music is because there was only Nintendo's own compositions, as well as a few public domain songs... who cares about "twinkle twinkle little star"? And another problem was there was no musical peripheral for it. No guitar accessory, no drums, no keyboard, no nothing. You do it with the wiimote/chuck. So that's two areas where it could have been hugely improved. If it was more like Guitar Hero or Rock Band and had popular modern songs people cared about and actual musical peripherals it could have been a very cool product.
So they should just copy Rock Band and not make a Nintendo game? If they did that, you'd complain that they didn't make a Nintendo game and just a clone of something you could already get with Rock Band.
Wii Music is so much different from other music games. It's not about trying to play songs that already exist, though you can do that. It's about taking that song and making it your own. It's about creativity and experimentation. It's about finding out if when you say you could make that song better, that you can back that up.
I feel like the reason Wii Music is hated is because nobody bothered to figure out how to play it. "You mean I don't get to play this licensed song exactly how it was originally done with a couple button presses? I'm not bothering with this then."
It's possibly one of the most ambitious games that Nintendo has done. That's probably the reason why it failed. Nintendo makes something new, but people hate. Then they complain about Nintendo never making anything new. The problem is that the "new" that they want Nintendo to make is basically clones of what everyone else makes.