Mary Jo Miller was just your common, everyday housewife before the zombie invasion came to her little suburb. Then she turned into your common, everyday zombie decapitator.
Mary Jo's story of everyday American life basically sums up the problems the novel has with the modern American lifestyle. Mary Jo is complacent when it comes to the zombie warnings on the news, since her life is too busy with such other worries as her kids' extracurricular activities, the fish's fungal eye, and her in-laws Christmas visit.
As Mary Jo says, "Yeah, we stopped the zombie menace, but we're the ones who let it become a menace in the first place" (9.5.4). Having becomes so preoccupied with her little world, she doesn't bother to research whether or not Phalanx will actually prevent zombification, and she doesn't realize the zombies are inbound until they break through her glass door. While her preparation alone might not have had much of an effect, the novel does suggest, that had all the Mary Jos of the world chipped in, we might have had an easier time of it.
Although the novel lays plenty of critiques on other aspects of modern life, Mary Jo's story suggests those issues suffer from a deeper problem: the average American lifestyle and the price of living in a democracy. Or as Mary Jo so nicely puts it, "in a democracy; we all gotta take the rap" (9.5.2).