Isn't all mindless activity performed with a clear conscience?
What I mean is, life is like a business. A good businessman carefully takes inventory, buying more of the hot movers and chucking the duds. In life, too, we should weigh each activity as to its benefits and drawbacks.
Does reading this book benefit you? If it relaxes you, that's a benefit. If it strengthens your bond with your kids because you read it to them, that's a benefit.
But if it wastes your time, that's a drawback. If it weakens your bond with your kids, because you hide in your room and read it instead of spending time with them, that's a drawback. Every activity has the "opportunity cost" of what you could have done instead.
So, while there's no specific prohibition against "wizard" stories, the question is how much time, if any, to spend on them?