What exactly do comet medals do?

i thought they brought out prankster comets, but i've found a few of them and nothing has happened yet. anyone know whats up

They unlock prankster comets for individual Galaxies, but not immediately, necessarily. Give it time.

They start showing up after you collect the first 4 starts of either Worlds 3 or 4, and I don't think they will show up in a particular galaxy until you get the Medal hidden there. Also, I think you need a certain number of Medals as well as the latter of the previous sentence until they appear.