He was a heavy young man, with not more than common sense; but as there was nothing disagreeable in his figure or address, the young lady was well pleased with her conquest. (4)

I don’t feel the narrator or Edmund are disparaging common sense. (:D) As I see it the problem with Mr. Rushworth is that he does not possess more than common sense. I interpret it to mean Mr. R has the common sense to make his way though the world without walking off a cliff but has a less than stellar understanding. He is not what Anne Elliot would describe as good company which is “clever, well-informed people, who have a great deal of conversation” (P, 16). As witnessed in Ch. 6 Mr. R is sadly deficient in the first two and while processing a great deal of conversation it is rather dull and as you suggest he is somewhat insensitive to how others respond to him and he appears to easily confuse himself. I also think he is being taken-in by Maria: