Michael Scott and The Peter Principle
It's been both mentioned and demonstrated in several episodes that Michael Scott was a really good salesman -- before he was promoted to manager, with a less-than-sterling result.
This is quite reminiscent of The Peter Principle, which (to quote the 1969 book of the same name) states: "In a hierarchy every employee tends to rise to his level of incompetence." Thus Dr. Laurence J. Peter, who wrote the book, would not be at all surprised to learn that a former top-notch salesman, when promoted to manager, was an embarrassment.