When the Champion Mastery system first came out, I was kind of intrigued. It was a way to show skill with a particular champion. However, it's implementation is flawed, and points essentially amount to whether you win or lose, not how you perform. The strangest part is that the system actually does seem to know when you're performing well, even in a loss. I've seen grade S to A in a loss, so something under the hood is working (one would think).
But it doesn't seem to have any bearing on how many champion points you are awarded at the end of a match. Most recently, I finished a game as Elise going 6/2/0 (our Caitlyn was constantly DCing, so we surrendered at 20:23). At the time, I had the most gold on our team, 3rd most gold in the game, and was responsible for 6 of 10 kills we amassed. I finished the game with an A+ grade, which I think that was reasonable. And out of curiosity, I hovered over the number of champion points I was awarded...
I got +37 for that game. A+ rank, only +37 points. Compare this to a game where I went 0/8/4 as Riven feeding my ass off. That game we still won, and I got over 1000 pts for a C performance. (By the way, If that was a C, what do you have to do to get a D?)
In the end, the only thing Champion Mastery tracks is how often you win the game with a particular champion, which I find shortsighted and foolish.