You can use this tool called My Avatar Editor to make and edit Miis. It runs on Adobe Flash, which was discontinued at the end of 2020. However, there are still a few ways to use it.
Set your computer's clock sometime before October 31, 2017. If you don't do this step, Adobe AIR will say the installer file is damaged.
Open MyAvatarEditor.air and install it.
Set your computer's clock to the latest time now.
You can now open My Avatar Editor at any time. Miis with the file extension ".mae" will now be associated with the application.
Flashpoint
You can use BlueMaxima's Flashpoint to use My Avatar Editor again. It's a project to preserve old Flash games. For more information, please see the official Flashpoint website.
Use in Browser
This won't load nor show up unless you're somehow using a legacy browser. (It will work with SeaMonkey)