It is possible to have 200 ranked games on a champion without gaining a single mastery point.
How?
Main Treeline.
Seriously, the fact that it doesn't grant anything makes zero sense. I would probably have ~20k more points on Zilean if 3v3 matches counted.
Could you imagine if CSGO only let you rank up while playing on Dust 2? What if HOTS only let you gain gold while playing on the Haunted Mines? What if OSU! only had leaderboards for half of its songs?
It just doesn't make sense.