For some reason, calling something simple is sometimes synonymous with calling it easy, or not very smart. But we want to call your attention to some simple (and awesome) things for just a minute to debunk this trend:
All simple. Also all awesome. So, when we say that the tone of Rats is simple, we mean simply that it isn't that flashy. You don't get huge vocab words that'll send you to your favorite online dictionary on every page, and neither do you get long flowery passages with tons of adjectives, and, millions, of, commas. Instead, you get straightforward language that tells a good story.
Sure, feelings can run high at some points in the book, like when Mrs. Frisby makes her speech about how the rats deserve a chance because "it would be the first time in all the world that intelligent beings, besides men, have ever tried to start a real civilization of their own" (25.11). But even at key, high point moments like this, the style of the novel never becomes anything but direct and to-the-point.