Each player has a set of nodes, and each node is a different shape used in the avatar screen. Usually, most nodes are hidden. This is what you would look like if you could select every different shape, and all of them be black.
I think this is a console command the host can type to make any player look like this:
findClientByName("PLAYERNAMEHERE").player.unHideNode(*);
tl;dr: the host of the server you're on is messing with you or someone used bot events to make you look like that.