The Sticker Comet is mainly a glowing, golden star that is pretty big in size. It also has smaller stars flying around it.
The Sticker Comet was first seen where Peach was celebrating its appearance during Sticker Fest. Peach explains how its origin is from the Sticker Star and can grant any wish. However, Bowser shows up and touches the Sticker Comet which causes it to split into six Royal Stickers. Bowser keeps one for himself and causes havoc throughout Decalburg. Mario and Kersti decide to go on a quest to find all of the Royal Stickers and restore the Sticker Comet. After Mario defeats Bowser, all of the Royal Stickers come together and the Sticker Comet is restored. Bowser attempts to touch it again but Kersti stops him in his tracks. Kersti then heads back to the Sticker Star along with the Sticker Comet.
The Sticker Star is a place where Kersti and the Sticker Comet are from and is what this game is named after. However, the Sticker Star is never actually seen but only mentioned by Kersti during points in the game. It appears that Kersti can only go to the Sticker Star if the Sticker Comet is present. It is assumed that the Sticker Star is located deep in space (possibly near Star Haven).