The phrase "mere decoration" means something that is used or added as a form of ornamentation or adornment, but has no real or significant purpose or function. Full definition
At one end of the band is a small, square tip, that looks like mere decoration. (techradar.com)
First - time smart phone users could easily excuse the clock on the main home screen as mere decoration, but if you touch it you can set the alarm, use a stopwatch, and more. (laptopmag.com)
He taught us that beauty in the liturgy «is not mere decoration, but rather an essential element of the liturgical action, since it is an attribute of God himself.» (faith.org.uk)