As a German, I might not have thought of that but would absolutely use it to translate Verschlimmbesserung. Does it sound "normal" enough to native English speakers?
"disimprove" works much better.
The similar word "unimproved" implies you did nothing to make it better, "disimproved" implies you actively made it worse.
But this is still just effectively adding "not" to the word. If you must join two words that are the antonyms of each other (like verbessern and verschlimmern), the closest match I can think of is the vulgar pun "fuxed", a blend of "fixed" (repaired) and "fucked" (broke). https://www.urbandictionary.com/define.php?term=Fuxed
Probably it would become "verskimbeserong".
Meanwhile I won't be able to remember that German word longer than it takes me to close this tab. Vershumthingsomething.