Definitely fiction. The storylines and situations in Diary of a Wimpy Kid are not based on real facts. It's a creative work designed to amuse and engage young readers through imaginative storytelling.
Well, to determine if the book is fiction or nonfiction, you need to look at elements like the plot and the nature of the story. If it's imaginative and not based on real life, it's likely fiction. If it's based on actual events, people, or facts, then it's nonfiction.
I'm not sure which specific 'wonder book' you are referring to. There could be many books with 'wonder' in the title. If it's 'Wonder' by R.J. Palacio, it's a work of fiction. It tells a heartwarming story about a boy with facial differences.