The phrase "mere prop" means a person or thing that is used only to serve a purpose or appear in a certain way, and doesn't hold any real importance or value beyond that. Full definition
And familiar character actors like Jeffrey Tambor and Philip Baker Hall (miles away from «Magnolia») again are mere props in Carreyland, even though they've proven themselves worthier than that. (hollywoodchicago.com)
Kids, parents and teachers are mere props for union leaders who line their pockets with worker - supplied gold and do whatever they can to maintain their unwieldy power. (californiapolicycenter.org)
Background characters are shot and maimed as if they were mere props. (parentpreviews.com)