How do I delete Miis from the Create Mii Fighter tool?
If I understand you correctly you want to know how to delete a Mii from your Switch console wide rather than just removing them as an option from the game. To do this follow these steps:
From the HOME Menu, select "System Settings."
Scroll down the options on the left and select "Mii."
Select "Create/Edit a Mii."
Select "Mii List." Choose the Mii character you want to edit/delete.
Select "Delete," then "Delete" again.
Nintendo Support, How to Edit or Delete a Mii
