Shark Fin Soup is a Crock Pot recipe introduced in the Shipwrecked DLC. It is made from 1 Shark Fin and 3 fillers. Because Shark Fin Soup has a higher priority than Monster Lasagna, multiple Monster Foods can be used as a filler, without the risk of getting Monster Lasagna as the result.
Real Shark Fin Soup is a classic, luxury Chinese dish. It's largely prized for the supposed health benefits that is said to be gained from consuming it. It's also been condemned for its effect on the ecosystem, with finned sharks being unable to swim and dying in the ocean combined with over-harvesting. This is why it causes naughtiness ingame.
The loss of sanity can also reference how large marine predators such as sharks have higher amounts of mercury in them. This is because they frequently eat other fish, thus absorbing their mercury.