How does the there of dont jugde a book by its cover is found in the novel?

By there you mean theme right? If so, it's found in the novel at multiple spots. It's like when she gets switched instead of being in room H-5 to normal classes. She starts off with a music class and other students who already attended the class sit with those who just joined (such as Melody and he other H-5 class-mates). Even though students see Melody in a wheelchair and make fun of her, a girl named Rose sits next to Melody and they start talking and later become friends in the book. That's one way of the saying "don't judge a book by its cover" is shown in Out Of My Mind.