Once the Mii Maker is open, you will be at the main menu. Here, you need to press the option QR Code/Image Options.
Now you will see two options. Of course, you do not need both QR options so choose the third one: Save Mii as Image.
Once you’re here, you will need to select your Mii and choose if you want to do just the face or the entire Mii. Meanwhile the latter option has a different name in both versions, it’s the same thing.
In the case of the 3DS, it will be exported once chosen. The Wii U version will offer an extra page instead, offering different faces or positions.
Remember you will not be able to export your Mii if you do not have a SD card inserted in the console. Meanwhile a SD card is included with the 3DS, you will need to get one for your Wii U. A micro SD with an adapter also works, but I don’t recommend it at all.
Congratulations! You now have the pic of your Mii on your SD card.