Are you guilty of this?
I'm very rusty with league atm and my main used to be AD Nidalee (never really played her AP) I know she's not really viable anymore, especially since she brings no CC to the team. That said I'm so refined and comfortable with her that I do better with her than any other champion by quite some margin at the moment. I still haven't picked a new top laner yet (thinking of Vlad though). That said because my skill level with her is so high above my skill level with other champions what happens when I do end up top lane and am forced to bring her out is I absolutely dominate the guy across from me. This is where the problem starts...
There is a reason I pick her, there's a reason I dominate with her. If you see an ally getting shut down by a crazy/stupid pick like AD nidalee when they're Fiora for example DON'T talk crap to them about losing to AD nid, don't insult them til they literally leave the game. The guy across from them picked that champion for a reason! Sometimes people have vastly different skill levels with different champions and can dominate people at their normal MMR with a champion they're simply much better at utilizing. I've had several teams now literally drive their top laner out of the game when this happens, it's not okay.
Also you know what? All of us also lose to that sometimes. Not only can it throw us off if we haven't faced the champion, but it's often possible that the guy across has faced your champion with that champion more times than they can count! I know every spell in most top laners kits and how/when they use them, I bet 90% of the people I'm facing have no idea how my cougars Q works.