Champion Mastery should be a reflection of a player's personal roster
Rather than using some arbitrary KDA win/oss measuring stick that is terribly skewed, Champ Mastery should compare performance of a player with previous performance of said player. That way a player who plays a certain champ will show that s/he has had experience playing this champion and can play it better than the rest of his/her champ roster. It will show, "oh, this guy plays these guys and is best at this champ"
The current system tells me either the player grinded 100 games on a champion they suck at, or played 5 games with a champion they are great at. I have no clue which one.
This will also help with players that only get a play a few games a week, so they wont have to grind out games and still be able to show people they have a favorite champion they are good at.