The phrase "mere backdrop" means something that is simply a background or secondary element, without any significant importance or impact. Full definition
The loot boxes of Battlefront II seemed to presage an industry that produced lavish video lottery machines, where all art had been sublimated into mere backdrops for mechanics whose sole purpose was the extraction of maximum profit. (theverge.com)
For one thing, rather than the hassle of trying to shoot on location in Sin City, Coppola and his crew built their own Las Vegas, a glitzy setting that sometimes serves as mere backdrop and other times is pushed to the foreground. (dvdizzy.com)
Occasionally, as with «Logan Lucky,» the setting is so crucial that it is not mere backdrop. (punchdrunkmovies.com)