[paint|gild] the lily

to unnecessarily adorn or decorate something that is already beautiful or perfect

The idiom "paint the lily" comes from Shakespeare's "King John" where a character uses "to gild refined gold, to paint the lily" to describe futile efforts to improve something already perfect. The term is used when people add excess embellishments to something good, in both minor and major situations, and in personal or professional contexts.

The design of the dress was already elegant, but adding more sequins and beading would just paint the lily.
The food at the restaurant was already delicious, but adding more spices and flavors would gild the lily and ruin the balance of the dish.
The garden was already beautiful, but adding more flowers and decorations would just paint the lily and make it look cluttered.
The speech was already moving, but adding more dramatic language and gestures would gild the lily and make it seem insincere.