Launching Miitomo after May 9 will not allow you to access its features, check your messages, or use your items. Nintendo has outlined that you can transfer your Mii to your Nintendo Account by linking it to Miitomo. It has also stated that personality and other information will not be carried over.
Furthermore, Sidekick Mii characters will be deleted as well. They can be saved as QR codes before the termination of service which you can then move to a Nintendo 3DS or Wii U. Miifotos will disappear as well unless they’re saved to your smartphone. Also, Miitomo-themed items used in your Nintendo Account or in Super Mario Run will not be visible after Miitomo ends.
While Nintendo hasn’t explained why Miitomo will be shutting down, the reasons appear obvious. The virtual avatars or Mii as they’re known began life on the Nintendo Wii and carried over to the Nintendo 3DS and Wii U. With the Nintendo Switch, this aspect of personalisation has been greatly deemphasised.
Throw in the fact that Nintendo is putting resources behind acquiring more content for the Switch, even visiting India’s developers to see what can be added to its eShop, and the popularity of its more traditional mobile games like Fire Emblem Heroes, and its easy to see why this is happening.