The mobile web version of Miiverse is more functional than the 3DS counterpart. Never heard Nintendo ever specifically refer to developing an actual native application for Android and iOS ... in fact, outside of the apps done by The Pokemon Company, don't think mainline Nintendo really acknowledges the existence of mobile platforms outside of their own.
Edit:
There's nothing that special about the the MK8 site. It's literally a glorified YouTube portal with some stat tracking. No idea what Miiverse is compared to ... Twitter without friends?
Kind of goes without saying that if you think the web version of garbage, the 3DS version has its own orbit outside normal space junk.