This could have as a consequence that somebody who tries a new champ and has a bad game might now get yelled at in subsequent games: have a bad day once, get punished for it indefinitely. You're not going to raise that publicly available KDA until you repeatedly pick that champ, have your team ask you to take something else when they see the KDA, then plow through enough games to bring to the KDA back up. Even then, you're just going to have a champ where you look okay, and you can still get people asking "Don't you have a champ where you're good?"
I think creating a system that adds pressure to only play champs where you've had a good record has a much higher cost than benefit. Also, I don't think it'd achieve the stated aim, since if someone wanted to troll, there's no reason it'd have to be on just one champ; you have over a hundred options to feed with if you're determined.