This would also invariably punish good supports, (who'd go something like 0/2/37) and reward the un-focused tanky DPSes and assassins who swept in and stole all the kills.
In short, it'd encourage kill-stealing.
We already have a system that automatically works to account for a player's skill: it's called Elo. A great player is NOT going to win all their games; real-world example: the Falcons and Texans represent the best of the NFL right now, and both have 2 losses to their 11 victories. However, if a player is genuinely good, all those matches that are "filled with scrubs on their team" will be balanced out by games where they get carried by everyone else... And a good number of games where they'll actually make their mark. Result? They'll win most of their games, and their Elo will go up.