Mrs. Jewls looked at the children. They were horribly cute. In fact, they were much too cute to be children.
"I don't believe it," said Mrs. Jewls. "It's a room full of monkeys!"
The children looked at each other. They didn't see any monkeys. (2.5-7)
Here's an example of how Sachar uses appearances for comedy's sake—mistaking a room full of children for a room full of monkeys is hilarious, as is the idea that the kids must be monkeys because real kids couldn't be that cute.
With two eyes she was pretty. With four eyes she was beautiful. With six eyes she would have been even more beautiful. And if she had a hundred eyes, all over her face and her arms and her feet, why, she would have been the most beautiful creature in the world. (11.1)
A very sweet description for Dana, isn't it? Especially sweet because sometimes kids are self-conscious about their glasses, but in this case Dana's glasses magnify her beauty.
Rondi was missing her two front teeth. And those were the most beautiful teeth of all. (13.1)
Poor Rondi is constantly driven to distraction because of her missing teeth, but they're her defining characteristic. In fact, when all the kids try to swap names and forget who everyone is, they figure Rondi out first because of the trademark gap in her smile.
The other two Erics were fat, and so everyone just thought that all the Erics were fat. (22.2)
In the chapter about the three Erics, Sachar writes some very sly commentary about how people tend to make generalizations based on appearances. Even though Eric Bacon is the skinniest kid in class, everyone thinks he's fat because the other two Erics are.
Allison was very pretty, so all the boys in Mrs. Jewls's class teased her, especially Jason. But Allison said, "Leave me alone or I'll knock your teeth out—like I did Rondi's." The boys didn't bother her after that. (23.2)
At Wayside, sometimes the sweet and pretty girls are teased for being sweet and pretty. Allison isn't the only girl who stands up for herself—Maurecia, also pretty, is described as being able to beat up any boy in the class. For these girls, pretty doesn't mean weak, does it?
Of course, his hair was still green. It always was. (29.44)
Here's a good example of how this book uses appearances in a comedic way—Sachar makes us think Stephen's green hair is for his costume, but really Stephen's hair is just green. This is the very funny punch line to Stephen's story.