Meh Heritage is an exclamation uttered after having your heritage mocked. It's to be understood in a joking manner as if you're either mocking the person saying it or if you're simply not taking it serious either because you know it to be untrue or if the original comment was a joke.
Person A "Your family is a bunch of thieves."
Person B "Meh Heritage!"
Taking a popular franchise like Star Wars or Doctor Who and making it boring by turning it into progressive propaganda device. Typically it features unrealistically competent "minority" groups like female lead of Force Awakens or Last Jedi, the character being so characteristic, simplistic and cartoonish that it generates its own popular names, like "Mary Sue" in this case.
Mary Sue got her Jedi training by Luke completed in one day. Man, what a meh-gutting!