The main themes of Atwood's "Edible Woman" are the search for oneself and gender roles and identity.
Marian searches for identity by observing those around her. She has trouble connecting with other women, and is critical of them. However, she herself cannot find her own identity through these other people or the view that others have of women. She looks inward in order to find out who she is, and while she doesn't quickly find an answer, she's on the right path.
Marian continually tries to identify her role as a woman. Everyone has different ideas of what that should be. Peter, her fiance, has created a false idea of Marian, and it is as if they are playing roles in a play. When Marian makes the cake-woman, it is a test for Peter to break out of those roles and eat the cake, which symbolizes the false image he has of her. When Marian eats the cake-woman, it symbolizes a breakthrough in her trying to get rid of these fake, empty identities.