Prankster Comets are unlocked after you have 12 stars. They make galaxies crazy in different ways depending on the color of the comet. There are 5 different types.
Speedy Comet
These red comets make you complete a level in a certain time. This time depends on the level e.g. Dino Piranha from Good Egg Galaxy. If you are struggling, try using lots of long jumps.
Daredevil Comet
These silver comets give you one life for a level. The challenge is usually to beat the boss, with the exception of Lava Spire Daredevil Run from Melty Molten Galaxy. This can be extremely challenging (take Bouldergeist from Ghostly Galaxy).
Cosmic Comet
These blue comets make you race another Mario (or Luigi). Cosmic Mario is easy enough (take Freezeflame Galaxy. Here Cosmic Mario follows a preset path, where you can take shortcuts). No, Cosmic Luigi shows us what hard is.
Fast Foe Comet
There are only two galaxies which have this comet ([../Beach Bowl Galaxy/]] and Toy Time Galaxy) which is just as well, because these yellow comets, as in the name, make foes move twice as fast. It's good if you can long-jump.
Purple Comet
You do this once in the Gateway Galaxy before having to beat the storyline to unlock this comet. The color need not be mentioned (it's obvious) and you have to collect those little coins. Sometimes you have a time limit.