Are the Mii's not considered user made content? In the upcoming Super Smash Bros', you can customize your own Mii to be a fighter and it looks like there's a whole community of Mii customizers [1] who make celebrities and the like.
Miis on their own are a bit weak because they're just a little display object. Their little social network Miiverse holds a fair amount of content though.
Also, Nintendo has said that they won't be allowing you to see customized Miis when fighting other people online in SSB unless they're a friend of yours, probably to prevent copyright (and objectionable content) issues.
There was a free channel/game thing on the Wii where they'd give you a theme ("Pilgrim", "Mean boss", etc) and you submitted a Mii that fit that theme. It was pretty neat to see what people came up with.
[1] https://www.google.com/search?q=mii+celebrities&tbm=isch