In-game posts now have their own community
Nintendo has released an update for New Super Mario Bros. U. The update, which changes the game's version to v1.2.0, now sends the Miiverse posts you make in-game after accomplishing certain goals to a dedicated New Super Mario Bros. U community.
The new community is designated specifically for these in-game posts, so players are only able to leave comments. The original Miiverse community for the game still exists for game discussion purposes.
The update should automatically download and install once you start up New Super Mario Bros. U from the Wii U menu.