Simple, Direct, Conversational
Reading Wonder is a little like getting to read excerpts from six different diaries. And though Auggie's story gets expanded by each different narrator, the narrators always start out by talking about something important to him or her. It's so effective: simultaneous plot and character development.
The first thing Auggie talks about, is—surprise, surprise—how his appearance impacts his life. Check it out:
And I feel ordinary. Inside. But I know ordinary kids don't make other ordinary kids run away screaming in playgrounds. I know ordinary kids don't get stared at wherever they go. (1.Ordinary.1)
Auggie doesn't mince words. He tells us, often with wry awareness, what's what. "My name is August, by the way. I won't describe what I look like. Whatever you're thinking, it's probably worse." Right out of the gates, Auggie lays all his cards on the table.
Via, on the other hand, is more complicated. She makes a big point of saying, "I'm used to the way this universe works. I've never minded it because it's all I've ever known. I've always understood that August is special and has special needs" (2.A Tour of the Galaxy.2). Via definitely loves her brother, and she does understand, but at the same time, the impact of his condition on her life has not been minor.
While swearing up and down how she totally gets it, Via is also quietly pointing out the unfairness of having had her healthy kid needs totally eclipsed by Auggie's medical needs. She explains:
I've gotten used to figuring things out on my own: how to put toys together, how to organize my life so I don't miss friends' birthday parties, how to stay on top of my schoolwork so I never fall behind in class.
There's nothing confusing about this passage: Via's had to take care of Via, as her parents have been maxed out taking care of Auggie.
Another way to think about the writing style in Wonder is like this: it's a story told by and about kids. Which is pretty much a guarantee that the tone is going to be pretty simple—the point in writing for kids is to have them enjoy reading, not despise it. And a great way to turn kids away from a book is to make it too complicated to really get into.
Plus, since Wonder is always in the first person, we get direct access to the thoughts and feelings of several of the characters—and the thing about thoughts is that they're unmediated, which makes them come across as pretty conversational. We don't know about you, but Shmoop doesn't think in fancy or formal language as a general rule—we're more like We're gonna eat that orange than We believe we shall commence consumption of that spherical reddish-yellow fruit—and neither do any of the kids in Wonder. Thank goodness.