Champion Mastery is flawed
So there is a massive issue with Champion Mastery at the moment.
It doesn't actually represent your level of skill with that champion.
Allow me to explain. You can sit on the summoner pad and have your team carry you to a win and still rank up to level 5. It will take you a while but under the current structure you have achieved mastery tier 5. How does this actually represent how good you are with that champion? It doesnt, and never will.
Champion mastery is ONLY a grinding platform. "We expect players to take 15-30 games to level through the first set of levels (1 through 5). We’ll be unlocking additional levels later." - rito mastery microsite
It should be changed to show your average rating across all of the games you have played. This would show your real level of mastery with that champion. If you played 5 spectacular games with Ryze and achieved an S across all 5 of those games then your mastery level is actually S. Not tier 5 or whatever.