Wii Music hasn't set the world ablaze. Still, it is an "evergreen title" — to use the proper Nintendo lingo. That, and Shigeru Miyamoto developed it.
One cannot dismiss Miyamoto's impact on gaming. He's responsible for so much of gaming's basic grammar. So even when one of his games does not garner the expected interest and acclaim, it is worth giving those titles another look. They very well could be misunderstood. From an interview with game magazine Edge:
What’s your response to some of the poor reviews Wii Music has received in the gaming press? Did you expect it to be misunderstood?
...My hope is of course that a gradually increasing number of people will get access to Wii Music and understand its fun nature. I really don’t think that it will have the immediate and universal appeal around the world at all [laughs].
I really appreciate that the gaming media has a different view of anything as new as Wii Music today — it’s simply symbolises how different and unique Wii Music is. To tell the truth, I have this big ambition for Wii Music, that it can eventually be something very influential so that it might be able to influence what music means in the world.
Wii Music might age well. It might be one of those titles that we come back to and are able to appreciate at all that Miyamoto is hinting at. Then again, it might not.
Miyamoto Unplugged [Edge Online]