Like most Nintendo games on mobile, Dr. Mario World allows users to connect their Nintendo Accounts to the game.
In the case of Dr. Mario World, linking your Nintendo Account allows users to recover lost save data or use save data on another device.
To link your account, press your finger on the gear icon to the left in the stage selection screen and then select “Back Up Save Data.” You’ll also be prompted to connect your Nintendo Account to Dr. Mario World after beating Stage 10. Press the button that says “Link” and you’ll be taken to a website and asked to sign into your Nintendo Account. You may have to use Google Authenticator if you enabled 2-step Verification, so make sure you have that downloaded and set up before linking your Nintendo Account. After that, your account will be linked.
According to the FAQ in the app, you cannot switch to a different Nintendo Account after linking one to the game. You can only link one Nintendo Account to the game data. You can’t unlink your game data and your Nintendo account once you have linked the two unless you delete your save data. If you delete the user data linked to your Nintendo Account, you can’t recover it again according to the FAQ. That includes data on purchased diamonds, coins, stage completion data, characters, etc. If you delete your Nintendo Account, then you can no longer use the game data for Dr. Mario World and you can no longer restore your data for the game.
If you previously linked the account to the game and want to do so again, you need to uninstall and then reinstall the game and link your Nintendo Account again according to the FAQ.
At the time of writing, it looks like linking your Nintendo account to Dr. Mario World only lets you back up your save data. You cannot currently earn points through My Nintendo by linking your account.
For info on how to link your Facebook or LINE accounts with Dr. Mario World, click here.