Graffiti is found all over Inkopolis, from every stage to the Plaza. It is shown in the form of Miiverse drawings, stickers and scribbles on the ground, billboards and on walls. They can be found on all stages and in Inkopolis Plaza. During Splatfests, they can be seen on the back of the two trucks Callie and Marie dance on top of, on banners, on neon signs, and in the sky possessing the form of what appears to be fireworks.
Some, if not all, graffiti in Inkopolis Plaza is implied to be made by jellyfish; as stated in a developers' interview, jellyfish are known to create a large amount of graffiti.[1]
Some of the stickers are Easter Eggs: which depicts other Nintendo characters, such as Bloopers or Octoroks.
Miiverse posts can be hidden via the Options screen. They will then be replaced by Splatoon-related placeholder drawings, the source of which is unknown.
If a player holds down when in the plaza while aiming at a Miiverse drawing, they can yeah it.
The background color of Miiverse drawings in stages is the neutral color of the two ink colors in the current battle.