From the Super Mario Wiki
Koopa Surprise is a strange, lumpy, yellow substance served to the prisoners of Koopatraz State Prison. It was only ever seen in The Super Mario Bros. Super Show! episode "Escape from Koopatraz". Little is known about it, except for the fact that according to Gramps, it looks and smells like Koopa. Although Koopa Surprise is edible, what it tastes like is unknown, although it's presumably unpleasant.
Mario started a food fight using Koopa Surprise during his stay at Koopatraz State Prison. This angered Warden Koopa, who opted to cut off the prisoners' food supply, which included Koopa Surprise.