How to Dump Mii Data from a My Nintendo Account
1. Go to the My Nintendo website.
2. Log into the account with the Mii you want to use in account creation.
4. Copy the text from the alert box that shows up.
This may not play well with accounts with multiple Miis, I don't exactly know. Here's an example Mii for Public Mii if you're having trouble: AwAAMHEkQZxnSUJ/lncYlZzmNflBbwAAAEBQAHUAYgBsAGkAYwAgAE0AaQBpAEBAAAAhAQJoRBgmNEYUgRITaA0AACkAUkhQAAAAAAAAAAAAAAAAAAAAAAAAAAAAABXD