This page will allow you to get some Mii images from a Nintendo Network ID, and that's it.
Miis are a copyright of Nintendo, I think. I advise you not to use these commercially or for profit or whatever. I'm not Nintendo, so I don't own Miis, please don't sue me.
Also, please don't use this as an API for your own site, get your own, lol. If you do, use the /seth/ payload. Thanks for understanding.