Hit the Print Screen button on your keyboard, open up Paint and press ctrl + V to paste the print screen. Then use the second button on the left side(a dotted box) and select your Mii. After that, hit Ctrl + X to cut out your selection, hit File > New and press Ctrl + V to paste your Mii. After that, just save it.
There is surely an easier way to do this, but I don't know it.
On a mac: Press Apple+shift+4. Draw a square around the area you want to save. Done.
The area is automatically converted into a jpeg, outputted as a file, and saved on your desktop. Its small things like this why I prefer OSX.