What about "worseprovement"
Alternatively "disimprovement".
Via https://dict.leo.org/forum/viewUnsolvedquery.php?idThread=49... and https://dict.leo.org/forum/viewUnsolvedquery.php?idThread=16...
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?
Alternatively "disimprovement".
Via https://dict.leo.org/forum/viewUnsolvedquery.php?idThread=49... and https://dict.leo.org/forum/viewUnsolvedquery.php?idThread=16...