German here. A "Verschlimmbesserung" is something that makes one part better and another part worse. If it only makes things worse you don't need a German word for it, you can just call it worse.
For things that were intended good, but ended up bad, there's an English idiom: the path to hell is paved with good intentions.
For things that were intended good, but ended up bad, there's an English idiom: the path to hell is paved with good intentions.