The green Dr. Mario represents Plague Knight, a plague doctor who wears a mask with a green color scheme.
The stage represents the dark atmosphere of the Explodatorium, an alchemy lab which serves as the location of Plague Knight's boss battle.
The music track represents the musical theme of the Mega Man series, which serves as the inspiration of Shovel Knight.
Dr. Mario starting off with a Bob-omb and the Bob-omb item reference Plague Knight's usage of explosive potions during his boss battle.
Other trivia:
This spirit's Fire & Explosion Attack ↑ ability further references Plague Knight's usage of explosive potions during his boss battle.
