As the story progresses, and Peter's parents rely on him more and more for help with his younger brother Fudge, I can't understand why he feels like a nothing. Of course, cute younger brothers and sisters get more attention from parents and others alike, but Peter should feel proud that his parents ask for help in dealing with his difficult young brother. He is asked to be a role model on several occasions, and he also gets a lock for his bedroom door. I suppose it's my adult hindsight that makes me feel like he isn't a nothing but actually quite important. The title is a good attention grabber for a potential reader, I suppose.
I think you're right...that the title is more about how he feels than how things actually are. (Not to mention that you're also right about it being an attention grabber.) Fudge's troubles certainly demand attention. On the other hand, if being a "nothing" means that you don't get into one startling predicament after another, maybe "nothing" isn't such a bad thing to be. I would way rather be a nothing than get into trouble like Fudge!
I understand your viewpoint, and perhaps Peter should feel this way (and does as he ages and is able to see through more experienced eyes). However, at this point in his life, he is "nothing" to his parents but a babysitter. He feels invisible, without a life of his own, no freedom to be and do what he is and likes because he is laden with the burden of caring for Fudge. The good kids never get the same attention that the kids like Fudge get...and they tend to blend into the walls because of it.