So you have made a backup copy of your legally owned Mario Kart 8 game from your Nintendo Switch, and want to play it on your Steam Deck. Once you have copied the XCI file you created into the EmuDeck switch roms folder and run it with Yuzu, you might have run into the issue of it prompting you to pick a Mii and just sitting there with a bunch of empty grey boxes, and never letting you past. Fortunately, someone else figured out how to get around this issue by providing some pre-defined Miis that you can install on your deck.
First, you will need to download the tar file containing the save data and copy it onto your Deck (DISCLAIMER: I did not create the files in this tar; I just repackaged them from the bis.rar file referenced on the page of the one who did create them). Once you have it there, you will need to go to the storage folder for EmuDeck. I installed mine on an SD card, so the path is there; if you installed it on the internal storage, you will need to find the appropriate directory.
Assuming you downloaded the file to your home directory (~deck), and installed EmuDeck on the SD card, these commands will install the files you need:
cd /run/media/mmcblk0p1/Emulation/storage/yuzu/nand tar zxvf ~deck/Switch-Miis-yuzu-storage-nand.tgz
Now, re-run Mario Kart 8, and you should see some Miis you can pick and be able to progress to play the game on your Steam Deck.