Backyardigan

Someone who joins a community/server and proceeds to not engage in the purpose of the community or server and instead shitposts or does everything except whatever the server or community is about. Different from a poser since they don't attempt to hide their lack of participation.

Tom joined the fighting game server, but instead of playing he spends all his time in the off topic channel talking about politics, shitposting, and porn and gets offended when someone calls him a backyardigan.