Champion Mastery is kill orientated
As a support player this is driving me utterly insane. I just got an A+ and two of my teamates got an S. The major difference being the number of kills my allies got and farm. As a support these aren't factors to us while playing the game. (we are told to not get kills or farm).
A major thing support players do is WARD WARD, this influence into score is incredibly low. 20 wards for a support should carry about the same weight as a kill, This is a full score letter difference for many supports.
Another thing is support farm is always the lowest (very obviously). With knowing that information supports typing/pings should count as farm.. (giving supports a CS score from not CSing). Many times as a support you end up being team leader and typically type and ping a lot to help get your team on the game page. This ability separates out good supports from great, but I'm fairly certain it's not in mastery.
The support role is vastly different in purpose from the other roles, the fact the champion mastery is treated the same doesn't feel rewarding.