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. Also, please check out this site for more Mii content.
Install for Desktop
It's possible to install My Avatar Editor on your computer as an application. Follow these instructions:
Download and install Adobe AIR. ( macOS / Windows) AIR is discontinued by Adobe, but it's still possible to use it.