Generally used in a professional setting, human clutter refers to a person who is so unproductive and utterly worthless that they are a detriment to the rest of the team. These people get in the way of other coworkers getting shit done.
The human equivalent to a bunch of plastic toys strewn across the floor.
Lindsay is a server at a busy restaurant. Lindsay makes messes all the time, but slacks off during cleanup. Lindsay stands around in the kitchen when she doesn't have any food coming up and should really be watching her tables. Lindsay is human clutter.