just played a game, 3/12 or something as yi, took zero towers, sated at 35 minutes, like 100 farm 40 minutes in, 1/3 to 1/4 as much damage dealt to champions as the other carries. got a c-, so apparently the D+ and D ranks are unreachable?
The lowest it goes is D-. It's going to depend on the champion. Yi is pretty feast or famine, so it wouldn't be that unreasonable for him to have to do significantly more poorly than safer champions to get a D-.
https://support.riotgames.com/hc/en-us/articles/204211284-Champion-Mastery-Guide#h1q4
i saw scarra get a D while playing Diana on his stream a few weeks ago, he went something like 0-16 with 40 CS in 20 minutes and said at the end of the game "this game was worth it if only for proving that it's actually possible to get a D in mastery scores"
