As a child, I had this book. And as a child, I liked this book. And as a child, I was not bothered by the fact that the animals in the zoo were in small cages, or that there were animals in the circus, or that what is called "violet" in this book is really indigo or just purple.
Now I have a child, and he likes this book. Maybe it's because the animal who want to join the zoo is unidentifiable. He is like nothing you've ever seen before, kind of a polar bear/wild cat hybrid with perfectly round, colorful spots that he can change on a whim. Or maybe it is the colors. Or the reactions of the boy and the girl in the story. It could be the text, which is simple without being boring, although my son can only read two words of it yet: "look" and "more".
Whatever it is about this book, I liked it when I was a child, my son picked it out and he likes it. Clearly the author/illustrator got the combination right.