Players could post to Miiverse's Splatoon community from the mailbox in Inkopolis Plaza, either with text or a drawing, both of which were created on the GamePad. Drawings could appear as graffiti or on billboards in the Plaza and in various Stages, and both drawings and text posts appeared over other Inklings in the plaza when approached. When zoomed in on or seen when examining another Inkling with , the player could "Yeah" a post by pressing .
Miiverse posts could be hidden via the Options screen. They would then be replaced by placeholder drawings, the source of which is unknown.
Due to the Miiverse shut down on November 7th, 2017, Miiverse Graffiti no longer appears in Inkopolis, and along with maps, such as Saltspray Rig and Ancho-V Games. This goes with interacting with other Inklings as well. Normally if the user wasn't connected to the Internet, other Inklings wouldn't have any Miiverse posts present. When connected to the Internet, other Inklings would have Miiverse posts and if they were drawings, they could be found as graffiti in Inkopolis.
Attempting to use Miiverse while it is closed, using the mailbox and trying to post something on miiverse will no longer work. The Wii U error message pops up and quotes,
|“||The Miiverse service has ended. Miiverse and any software features that make use of Miiverse will no longer be available||”|
It's unknown why Nintendo never removed the Mailbox from Inkopolis and can still be used, but the message blocks players from posting.
Since Miiverse drawings can be found in Saltspray Rig, for a strange reason, Miiverse drawings are still visible on the neon posters in the mission in Octo Valley Octoling Onslaught ~Rumble at the Rig~. 
- Miiverse was split into three regions: North America, Europe and Oceania, and Japan. Players from one region would not see posts from another.
- Miiverse posts could be un-yeahed by going into Miiverse.
- Some, if not all, graffiti in Inkopolis Plaza was implied to be made by jellyfish; as stated in a developers' interview, jellyfish are known to create a large amount of graffiti.
A Miiverse drawing as graffiti at Moray Towers.
Another Miiverse drawing on a sign at Saltspray Rig.
Japanese Miiverse postings during Splatfest.