The Last Lombax
Member
Would I lose my friends list and other data if I uninstall the app?
I think if you link the app with your Nintendo Account you won't lose your data.
from Miitomo FAQ:
What happens if I delete or uninstall the Miitomo application from my device by mistake?
You will no longer be able to use Miitomo unless you reinstall the app. Additionally, unless you've linked the app to a Nintendo Account, all your data (including your Miitomo coins) will be lost, and you will have to start again from scratch after reinstalling the app. Please take care to avoid this.