The whole novel has a legendary, mythical tone to it: the narrator describes Maniac's actions as though the boy were a figure from tall tales, like Paul Bunyan. So, we can find hyperbole featuring throughout the story, adding to the humor and the interest of the events. Let's check out some examples from the chapters you mentioned.
From Chapter 23:
When Maniac finally forced himself from the shower, he found the old man waiting with clothes. Grayson's clothes. "I called the U.S. Army in to haul them buffalo rags away," he said. "They come in with gas masks on, and they used tongs to pick 'em up and put'em in a steel box, and they took the box away to bury it at the bottom of the first mine shaft they come to."
That's Grayson talking, hamming it up as he describes what had to be done with Maniac's stinky clothes. The boy had been staying in a pen with some buffaloes, so he really did smell like one when Grayson took him in! The old man's hyperbolic description adds humor to the story.
From Chapter 25:
Sleazy hotels. Sleazy buses. Sleazy stadiums. Sleazy fans. Sleazy water buckets. Curveballs and bus fumes and dreams, dreams of the Majors —clean sheets and an umpire at every base.
As we read this this exaggerated description of the stories Grayson shares with Maniac, we start off believing in its truth—sure, hotels and perhaps even buses can be sleazy—but then the hyperbole kicks in and we just laugh at what a "sleazy water bucket" would possibly be like.
Here's another example from that chapter:
The batters were teeing off as if it were the invasion of Normandy Beach.
This means that the baseball players were taking the game as seriously as a major war. That's definitely a stretch: an instance of hyperbole that adds drama to the story.
From Chapter 27:
"I'm reading!" yipped the old man. His smile was so wide he'd have had to break it into sections to fit it through a doorway.
The narrator's hyperbolic description of Grayson's excitement as he reads his first sentence conveys the extent of his joy. As readers, we experience his pride and happiness as if it were our own.
You'll find this example in Chapter 29:
Maniac thought of Thanksgivings past, of sitting around a joyless table, his aunt and uncle as silent and lifeless as the mammoth bird they gnawed on.
As he thinks that his relatives were so quiet and unhappy that it's as if they were actually dead, Maniac further characterizes this couple, helping readers understand why he absolutely had to run away from their loveless home.
Here's a hyperbolic description of Maniac's room in Chapter 30:
Santa's elves themselves would have felt at home. Strings of popcorn swooped across the ceiling. Evergreen branches flared at random, dispersing their piney aroma. Wherever there were a few vacant square inches, something Christmassy appeared: a matchbox crèche, a porcelain Santa, a partridge in a pear tree.
We can tell that this is hyperbole because, according to the narration, pretty much every inch of the room had holiday decorations in it. That's almost impossible! The effect of the exaggeration is that we understand the happiness and excitement felt by both Maniac and his friend Grayson as they share the holiday.
Finally, there's some hyperbole to be found amongst the sober sadness of Chapter 32, but it's rather rude:
"I'm freezing my cochongas off," a pallbearer announced.
He means that it's so cold that some of his body parts are freezing and falling off. Of course, that's an exaggeration, and it serves to show how crass and insensitive the pallbearer is, in contrast with the serious grief that Maniac feels.