There is a difference between not being good at a champion and not doing well and simply just making stupid decisions and not trying.
Whatever the case was, I'm just throwing that out there. We've all gotten a bit flustered while playing an ARAB because of a loss from a player simply just sucking.
The people who leave or seriously rage when they get certain champions are the ones who are easily at fault, though. I know when I get Riven I will suck on an ARAB (also because frequently when I get Riven there is no tank on our team) as I play her as a damage dealer, and thus I make sarcastic comments such as *rages over riven* during champion select, however I won't leave out of good sport and respect to everyone else.