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 Mario characters perfectly.
Use "shading" for the usual style with gradients and "flat coloring" for flat color
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.
Retrained with better tags in V2, you no longer need to specify the character's appearance, it just knows to do normal clothes by default :D