Marian was undergoing a massive and very intense inner change. She was no longer acting in accordance to what she had decided to be the journey of her life. She was now a walking and talking reaction. Hence, her brain was no longer waiting for her commands, but commanding her by creating this anxiety over food, which was merely Marian's way to express her fear of her own person being consumed by social and marital expectations.
Hence, hers was not necessarily a spiral-down journey into despair. Atwood wanted to leave Marian the way we left her at the end of the story to show that not only was Marian eating again, but that Marian has finally found herself, has taken control, and has found what she was she was looking for. However, Atwood made it look comical, showing Marian chomping down a cake in the shape of a woman, in defiance of the mandates she feels that society has placed over women.