It's easy in retrospect to class Mendy as a bad signing, but I don't recall anyone saying that in his first few games for us when he was getting lots of assists and creating even more chances. Ignoring the off the field issues for a second, it was the knee injuries that did it and from when he did make a short comeback, it looked like he couldn't run properly and I'm not sure he'll ever be a top level footballer again, but the sort of thing that can happen to anyone.
Then there's the off the field stuff and without suggesting anything he did was illegal, it was questionable behaviour for a professional footballer, even an injured one. Won't be the only player to not behave in the best way off the field (not even the only City fullback!) but doesn't suggest dedication.