Farm should not be this important in determining mastery rankings
I've had perfect games (in recent memory, 5/0/7 volibear and 18/2/something renekton) where I've gotten low mastery grades (B+ and A+, respectively) due to farm.
Currently (this is just speculation), I think Riot determines mastery grades according to your gold income/minute. This theory is roughly in line with my experience of 15 cs ~ 1 kill in terms of mastery grade.
Kills are worth far more than 300 gold, however. A first blood invade is 400g. A kill in early lane denies your opponent around 2 waves of CS on top of getting you 300g. A kill at the 35 minute mark could mean a free baron. In every stage of the game, a kill is worth far more than 15 cs, and yet the mastery score system seems to weight a kill at 15 cs.
Kills and assists should be more important in mastery grade calculations than they are now. Just my two cents.