Such a designation possibly indicates the unattainability of all that Madame Ratignolle represents. Further, because Edna was not raised Catholic (a religion that places a great deal of importance on the Virgin Mary), her view of the Madonna is from the perspective of an outsider — one who was not brought up to value a supreme mother figure.
Edna herself is no Madonna: When her children appear on the porch, she "sought to detain them for a little talk and some pleasantry" as if they were social callers rather than her small boys. In contrast, Madame Ratignolle showers her clingy children with "a thousand endearments" while cuddling the smallest in her arms. Note that immediately prior to complaining of feeling faint, she neatly collects her sewing work and supplies, rolls it all together and pins it "securely" — hardly the behavior of someone feeling faint. Madame Ratignolle takes advantage of her society's view of woman as helpless to exert a kind of power over others, power she cannot exert directly while still remaining within her culture's bounds of propriety.
