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I'm going to be a voice of dissent, but the Game being Quidditch isn't really meta, neither is Petra being a Potter fan.
Meta, to take a simplistic view is when something comments on itself. Zombieland is meta because it calls out the zombie rules and makes the Romero references, essentially calling attention to the fact that Zombieland itself is a zombie movie.
Petra's fanboyism is just a character trait, it says nothing about the larger whole of the world, even with the consideration that the GFL is a sports series, the parallels to Quidditch (in either form) aren't really there, so there's commentary aspect to this.
Though the old trope is for characters to reference Shakespeare or Wilde, those references in themselves almost becoming a living thing - there'll be some kids who only know Gilbert and Sullivan by playing Mass Effect.
That's one reason why some people shy away from more contemporary references - the chance that it will date a work without the emotional impact or resonance. If Xena had lasted six episodes, it wouldn't be an easy reference for someone to make, or if BSG has only ever had the initial miniseries without getting picked up, frak wouldn't be in wide use.
TLDR - characters can be fans, it doesn't make it meta.
*prepares to get squished by plaid tanks*