Meg is a key part of the familial continuum, I think. She’s smarter than Chris and Peter, and self-aware of what a weirdo she is, but she’s not as smart as Lois, so she’s unable to hide her weirdness and basically goes through life with a “Kick Me” sign on her back.
Her interaction with the family just taps into a deep vein of FG humor: denying sitcom expectations. When Chris takes a cheap shot, that’s expected, because that’s what siblings do. But Peter or Lois is supposed to step in and be the adult, and make Chris apologize. [Studio audience]Awwwwww.[/studio audience] But they don’t, they pile on. And sometimes they’re just flat-out mean in a way even Rosanne never was. Lois reading aloud from Meg’s diary and mocking it was one of the greatest so-painful-you-must-laugh moments in the series. They couldn’t beat up on Chris that way, because for one thing, he’s too easy a target, and for another, he wouldn’t be self-aware enough to take it to heart like Meg does.
I think Meg, who is, as you say, the sanest of them all, is in part meant for the audience to identify with. She reacts to the family’s weirdness the way we might. But at the same time, she’s the embodiment of everything that us geeks hate about ourselves, and especially our adolescent selves: the awkwardness, the inability to fit in, the eagerness to please. By laughing at her, we can distance ourselves from that time of our lives and those parts of our personalities.
This is especially true because there’s a certain inevitabilty in the Meg-plots. When she gets kicked, you can see it coming from a mile away, because she had it coming. You see her firmly affix the “Kick Me” sign, and you have time to steel yourself in anticipation of the next humiliation. After the blow has fallen, she invariably whines about it, or bawls, or both—never any pluckiness or strength of character that might make us rally to her. She’s a punching bag, and you don’t care about her.