Currently Online 
I think it will do well to be honest. Miis are for casuals and casuals are all on the iPhone/Android, so that actually works for Nintendo rather than against them for a change.
And it's free to start, I'm guessing it will have a lot of Nintendo loot/items you can obtain (ie: pay for) so that should work.
I just wonder how much "meat" there is to the game, as in will it be something that people will want to use everyday. If there's not much to do with it after the initial novelty wears off, then it won't work, but I would imagine there would be some (?) gameplay elements?
Tomodachi Collection is a big hit in Japan and was an ok sized hit in the US/Europe, so if its kinda like Tomodachi Lite with Streetpass style stuff, I could see this doing quite well.
I get the feeling an app that can use Amiibo toys is coming too.
