I don't think anyone thinks managers don't matter, just that over 162 games, they probably have very little overall impact on a teams win loss record, something like 3-5 games tops, and the difference between the best and worst managers is nowhere near what people think it is. Once a guy gets a managerial job, it's pretty safe to assume he knows just as much about baseball as any other manager.
The real talent in a manager is how he manages the egos and the psyche of his players. Theres probably truth that certain managers don't belong in certain clubhouses, guys like Cito who was known as a veterans manager probably wouldn't get along well with teams like the Cubs, made up primarily of younger guys.
Theres probably also truth that some managers don't manage the post season well, like I and several others are seeing in John Gibbons.