For starters, the best way to learn to read is phonetically. Teaching kids to read by rhyme, and indeed with the use of nonsense words, actually limits vocabulary and the development of reading proficiency.
Then, too, his imagination was not the imagination of a Tolkien or C. S. Lewis. They developed worlds where the interaction of the characters told compelling stories. Suess just made up things which matched his rhyming scheme. "What would you do if you met a Jibboo?" or whatever that creature was, cannot really inspire anyone, even a kid. With no further extrapolation, it's just dumb.
But perhaps the way in which his writing was most awful was in the lessons it presumed to teach. Take 'The Cat in the Hat' for example. Basically, this cat half destroys a house while the children's mother is out, miraculously cleans it up, then the tale ends encouraging kids to be dishonest with their parents. That's not a lesson we ought to be teaching our young, especially in this age of moral relativity.
Or The Butter Battle Book, written during the Cold War, which essentially morally equated the United States and the Soviet Union by demonstrating our relationship as an absurdity: they simply butter their bread on the other side, you see? That's nothing short of simplistic, mindless hogwash.
That Mr. Geisel has had such a profound effect on our reading habits is not a good thing. It is high time to remove his books from our shelves, and give kids better reads. At that, they may actually learn worthwhile lessons.