Flashing masteries, like Candurill said, means nothing...just means that you played lots of games with that champion...doesn't mean you're good at them. Plus not to mention no one likes a toxic, jerk of an opponent that constantly flashes their masteries....that's not good sportsmanlike conduct and I wish riot actually punished people for constantly spamming it as it IS harassment and unsportsman like conduct.
Downvoted because I think it's such a bad idea and will make toxic players even more so by saying "hey look at me I've played X amount of games".
Best feeling in the world: completely destroying your opponent in lane even though they are so sure they are better then you and keeps flashing masteries