Alan Garner is a British fantasy writer who quit his studies at Magdalen College, Oxford, shortly after C. S. Lewis left for Cambridge. Garner claims his stint at Oxford left him with an appreciation for academic rigor for life! But judging by his criticism of The Chronicles of Narnia, I think he should have stuck it out and learned how to do sound criticism and use words more correctly:
"They were, in my opinion, and remain, nasty, manipulative, morbid, misanthropic, hectoring, totalitarian and atrociously written."
Nasty? I can hardly imagine what he means. Some claim Lewis is mean to Susan. Maybe he doesn't like satire: there are some satirical passages that may scrape his skin. But no one gets cannibalized or baked in an oven, as for instance in the Brothers Grimm. Judgement is a normal and welcome part of children's stories.
Manipulative? Pullman makes that accusation about the Crucifixion and Resurrection scenes: I suspect Garner is echoing his more popular contemporary. This is a charge I will refute.
Morbid? Was he reading the standard edition of Narnia? "Light-hearted" or "gentle" would seem more accurate. Lewis doesn't, like Pullman say, have children tortured and get their souls ripped from their bodies.
Misanthropic? Lewis shows he hates and avoids people in those books? Even though they are full of people, many of whom are heroic, and kindly anthropomorphized creatures? (Nikibrik might be described as misanthropic: not Mrs. Beaver!) I don't think the ratio of villains or scoundrels to heroes is especially that high in these books. And Lewis himself was an extremely kind man, with legions of friends. Another really weird accusation.
It's also hard to guess what Garner means by "hectoring." Perhaps, from the fact that he describes the Bible as "myth," that he objects to the Christianity of the Chronicles. Hectoring may mean he dislikes its moral component. That aspect seems healthy and human, to me.
Totalitarian? Aslan is not a very good totalitarian ruler: he doesn't even show up in Narnia for centuries at a time. Peter and the other kings and queens seem to take a laissez-fair approach to ruling. Caspian liberates slaves, and even school-children, and Rilian liberates the gnomes. Indeed, the job description of "king and queen" in The Magician's Nephew is basically "work the ground," "settle civil disputes fairly" and "lead the army when an enemy attacks." In the American context, we'd call that "conservative" or even "libertarian."
And Garner is a Brit, where kings and queens have ruled for more than a thousand years, usually without being "totalitarian" about it. A writer should use words more carefully. He appears to have whipped this one out at random, to beat someone he doesn't like, lashing him with the wet noodle of an ill-chosen pejorative.
As for "atrociously written," I would say "somewhat casually written." He was writing for children. There are errors, sure: he was not a perfectionist, like Tolkien. But obviously books that sell 100 million copies, and are re-read a dozen times by a grownup-scholar like myself, milking great and repeated delight out of them, and deep insights too, cannot simply be dismissed as "badly written."