That's honestly just a nice detail, not the primary reason I consider the game dark. That would be the context given to the Shadow Queen, her reign of terror in the past and how the Pit of 100 Trials was a dungeon she used to torture people and feed her pet Bonetail, and how some of the partners have emotionally-heavy conflicts involving coping with loss of a loved one (Bobbery) and domestic abuse (Vivian), which is unusual for a Mario game but very welcome as it made me care about the characters. It's not dark as in "here's someone being stabbed to death on screen" it's dark as in "this world has a twisted history to it and characters are not safe from emotional trauma that can be felt by the player"
