Dr. Seuss was a serious doodler. But he didn't just doodle his way into kids' hearts. He also drew political cartoons for newspapers and magazines. (Fair warning: his cartoons are a wee bit racist to our modern eyes.)
The Butter Battle Book looks a little more like these political cartoons than it does most of his other children's books. How so? Well, the images might be exaggerated for effect, but they're also recognizable. For example, compared to some other Seussian characters, the Yooks and the Zooks look pretty humanish. They're kind of like people, just with beaks.
Seusspert Philip Nel also suggests that, in The Butter Battle Book, "even if [Dr. Seuss's] line lacks some of the effortless feel of earlier work, the book's art gains strength in its willingness to pare down. Just as the culmination of the book's weaponry is destructive power in its most condensed, so the emotional power of the book's art derives from its conciseness." Translation: sometimes it pays to keep things simple. As readers, we're able to focus more on the message and less on the décor.