I personally do not think there is any use of allegory in this story. Let us remember that allegories are stories where almost every character, action and event stands for both itself and some other, more symbolic meaning that points towards some kind of moral lesson or message. A good example would be George Orwell's Animal Farm, or "How Much Land Does a Man Need?" by Tolstoy.
I personally don't think we can read this excellent novel by Atwood in this way. Instead, I think Atwood uses symbolism to convey her message, in particular with the cake-woman that Marian makes at the end of the novel. Let us remember that this novel is above all based on the search for identity of what it means to be a woman, and throughout its pages Marian struggles to define herself and finds all sorts of models and examples of femininity that she rejects. The symbolic cake-woman therefore represents her defiant desire to reject and spurn the various feminine identities that are explored throughout the novel. Note the way that the cake-woman is described as being "an elegant antique china figurine ... its face doll-like and vacant," which clearly responds to many views of femininity that prize outward beauty over inner agency and intelligence. An important event in Marian's search for self is when she begins to eat the cake-woman, regaining her hunger, clearly symbolically rejecting such views of femininity as have been expressed in the novel. Ironically, when Ainsley accuses Marian of rejecting her femininity by eating this cake, Marian replies by saying: "Nonsense, it is only a cake."
Therefore, I don't believe this book uses allegory in making its argument about the various femininities that society has constructed and imposes upon women. It does, however, use symbolism, and the best example of this is the cake-woman at the end of the novel.