The general consensus among scholars is that The Lady Down Below is an allegorical figure representing traditional female values. The Lady Down Below, Marian and Ainsley's landlady, is deeply hostile to the consumption of alcohol, which she regards as unladylike, frowns upon relationships with men outside marriage, and above all else is concerned with maintaining the moral purity of her home.
At the time The Edible Woman was written, the status of women in Atwood's native Canada was undergoing quite a considerable change. A whole new generation of women was rebelling against traditional gender roles and sought to take greater control over their bodies and their lives. The character of Marian can be said to represent this radical change. She rebels against the limited role that patriarchal society has set down for her. In doing so, she's openly defying the traditional values represented by The Lady Down Below, who has internalized the restrictions placed on women by the society of men.
The fact that Marian's landlady lives "down below" is suggestive, as it strongly implies that she's somehow beneath Marian in terms of gender consciousness. And given her firm attachment to the old ways, it's more than likely that The Lady Down Below will, in metaphorical terms, remain beneath Marian as the latter tries to break free from the confines of patriarchal society.