I guess it depends on how you use the app. I comment in random people's answers if I find the answer interesting or funny, and I have some laughs with them, so I really don't mind having a lot of people in my friend's list. I basically use it like a more cheerful Twitter. And because of Miitomo being a cheerful environment it encourages me to comment random people's answers, which is something I don't usually do on Twitter. I'm really digging the app. The only problem I have is that I don't know how to fix my notifications, since they pop up randomly and, in ocasions even 1/2 days after some person posted in my answer. Obviously Miiphoto and male fashion (in my case) are awesome and fun in the app too.
I'm not entirely sure on how some things work. For example, can I visit other people's places or my Mii does it randomly when he feels like it? Because I guess that the people your Mii is "thinking about" pops up when he went to visit somebody so he got some info about the other person.
