Wasn't happy with the existing Illustrious LoRAs for this on this site, so I trained my own. Manually tagged for characters, so it can recognize most Mario characters pretty well.

"shading" for the usual style with gradients and "flat_coloring" for flat color

Note that Illustrious does struggle with generating Princess Peach/Rosalina/Daisy's lips, I suggest running a face fix if you encounter this

The higher the CFG/strength, the closer it will be to official art :)

This not only works for mario characters but can be used to generate other characters in the Mario style