The lima beans would have cured Camilla even if she hadn't liked them.
Maybe the morphing wasn't really a result of not being true to herself like the readers are meant to think, but instead is a strange disease and lima beans are just the cure.If it weren't for the old lady who give her Lima Beans and cure her, Camilla would soon turn into her hometown, then the country, then the whole country and soon the whole earth, and then the entire universe.
Everything except the ending is All Just a Dream.
The morphing and other bizarre events never really happens. It was just Camilla's anxiety dream about what would happen in her first day of school. The dream ended after Camilla eat the Lima beans and was cured. She woke up and realised the morphing never really happen to her relief but she did learn from the old lady in her dream that it was okay to be herself. Taking the old lady's advice, she went to her first day in school and stay true to herself by eating Lima beans in front of everyone.The book's world was created Last Thursday.
The world isn't really governed by any physical laws; anything can happen at random. Everyone's memories from before the world's creation are fake, and outside the town there is just a neverending expanse of primordial chaos. Camilla's transformation is the spark that makes people realize this.