I don't care if you are the worst player in all of League of Legends
I do care if you leave a game early on because you died a couple of times and don't come back because you gave up. Even a horrible player can still be useful, but leaving a game makes everything difficult. League of Legends is not just about KDAs, but also objectives and farm (unless you are a support) can turn that difficult game into a win!
Quitting when things get slightly difficult makes you a worse player and lets your other 4 teammates suffer, it's not cool and you won't go far with that attitude.