Also the most 1 run losses in MLB (though have also played the most 1 run games overall).
That said, offense is as big of a reason for that as bullpen, if not bigger. The Cubs didn't hit for basically 2.5 months. Way too many games the Cubs could have pulled away in but didn't causing the pen to eventually blow it.
Fixing the offense is the number 1 offseason priority for me, ideally by finding a way to acquire a star middle of the order bat. Easier said than done but if they can't find a way to do that, their ceiling is always going to be limited IMO.