Why do you get like 100 points if you lose and 1000+ points if you win? I've gotten an S+ in games where I've lost, and I surely deserve more than 150 mastery points. If I look at my champion collection, I have more MP with Ashe by playing ONE GAME and winning (not even playing well; got like an A- or B) than Vel'Koz, a champ I have 5 games on, gotten S twice and A's/B's the other times.
Your mastery points game should be based on your score, not whether you win/lose. Why? Because league is a TEAM GAME, meaning that having high mastery on a champion doesn't necessarily mean you'll win. Since wins/losses, being team based aren't DIRECTLY related to how good you are with a champion, you should gain MP based on your performance in game.