>attempted improvement that only makes things worse
Unless I'm mistaken, Russian has a wonderful idiom for this: "bear job"
It‘s „bear service“, and context is slightly different. You do bear service to someone, and Verschlimmbesserung is on something. Both signal intent to do something good, but doing bear service is an act of screwing up while doing that, no prior condition required, and Verschlimmbesserung is the outcome of such attempt applied to something that was ok before.
That actually also exists in German! Bärendienst, although it is not 100% the same as Verschlimmbesserung.