Thanks for the review. From the pages you included if seems like there's an element of black comedy in the story, which I like in horror.
As for the title: if I understand correctly, and I'm not Japanese so I probably don't get all the nuance, Japanese "common sense" refers to the social rules, norms and etiquette that people are expected to conform to. It's "common sense" because it's believed that people should know these rules from an early age and not have to think about it when interacting with others. It seems that the monster Takahashi can imitate this "common sense" perfectly while the human Itou struggles. In "The Summer Hikaru Died" "Hikaru" has its memories, inherited from Hikaru, to enable it to fit in but still doesn't understand, at least in the beginning, how it feels to be a fragile, mortal. Interesting topic for a compare and contrast essay!
Edit: I should probably point out that my use of "common sense" is not intended as derogatory "scare quotes". It's just to show that the phrase is being used in a special way.
Edit 2: To be on the safe side, added tags for possible "The Summer Hikaru Died" spoiler. I'm new here and don't want to get off on the wrong foot!