Because he is essentially homeless, Maniac Magee pretty much eats whatever he can get his hands on. His most favorite food is Tastykake butterscotch Krimpets, and, oddly, he is allergic to pizza.
Among the things that Maniac eats in the book are spaghetti at the Pickwells', "carrots, apples, and day-old hamburger buns" at the deer shed at the Elmwood Park Zoo, and a bite of Mars Bar Thompson's candy bar which was offered as a challenge. When he initially is living with the Beales, Maniac tries pizza for the first time in his life, and breaks out in "round, red blotches" all over. At the East End Fourth of July block party, he feasts on grilled chicken and ribs and sweet-potato pie, and at Cobble's Corner, he has orange soda and is introduced to what would become his favorite food, butterscotch Krimpets.
When Maniac, near starvation, is rescued by Grayson at the zoo, the first thing he eats is some chicken noodle Cup-a-Soup. He also has a zep, which is like a hero sandwich, and more Krimpets. When Grayson takes the unbelievably hungry boy to a diner, Maniac "wolf(s) down meatloaf and gravy, mashed potatoes, zucchini, salad, and coconut custard pie," and one morning, in the bandshell, the old man brings him "an Egg McMuffin and a large orange juice." On Thanksgiving, Grayson and Maniac cook a magnificent dinner on two hotplates and a toaster oven. Their bill of fare is "a feast of roast chicken, gravy, cranberry sauce, applesauce, SpaghettiOs, raising, pumpkin pie, and, of course, butterscotch Krimpets."
I believe what you are referring to is when Maniac is at Amanda's home and her parents order pizza. Maniac has never had pizza before and he has an allergic reaction to it. He ends up getting a rash and swells from the food. The Beales are amazed. They had never heard of someone not having eaten pizza, let alone being allergic to it.
The pizza is just one of the new and unusual experiences Maniac has after running away from his aunt and uncle's to find a home of his own where he feels that he fits in well. His stay with the Beales seemed initially to be going well.