Whether we're talking Yertle, Gertrude, or the animals of "The Big Brag," these crazies are all classic moral characters who behave foolishly, just so you—yes, you—can learn a lesson.
We can see this tone in the way Dr. Seuss gently mocks his characters throughout, without ever being too cruel or condescending. After all, he wants us to identify with them, and we don't want to think that Dr. Seuss is mocking us. He couldn't be, right?
When Yertle, for instance, says things like, '"Oh marvelous me!"' (Yertle.29), and '"Turtles! More turtles!"' (Yertle.48), and '"I shall not allow it! I'll go higher still!" (Yertle.78), Dr. Seuss is showing us just how ridiculous of a character Yertle is. And when Seuss uses words to describe Yertle like, "bellowed and brayed" (Yertle.48), he's telling us how ridiculous Yertle is, too.
But, like we said, the tone doesn't get all lecturey. Instead, it stays silly, goofy, gentle, and fun. That's where words like "howled" (Yertle.71) come in. Yertle is ridiculous, sure, and he's certainly ridiculously frustrating. But it's hard to get too upset when he's howling all over the place. You'd better bet that word choice was intentional.
The same goes for "Gertrude McFuzz." Where Seuss could have said things like, "A bird that was way prettier than Gertrude," or "Gertrude's unacceptable, infantile behavior," Seuss instead uses words like "fancy young birdie" and "pouted" and "tantrums" and even the name "Lolla-Lee-Lou" to show us how silly this whole thing is. But he doesn't forget to spare some words that remind us to take things seriously, like, "Such talk! How absurd! Your tail is just right for your kind of bird" (McFuzz.18-19).
In "The Big Brag," there are silly, boastful words all over the place. Check out the first sentence, when we're told that the "rabbit felt mighty important that day," and again in the third when we get, "He felt SO important" (Brag.1,3). In fact, a fair portion of the words in this story are somehow related to the term "brag."
We see this bragginess the most when the characters get into their spitting matches, saying things like, "Pooh!... Again, I say Pooh!" (Brag.14). The lesson conveyed in the tone here seems to be: if you get overly obsessed with comparisons, you'll end up saying ridiculous stuff like this.
So just be you.