I don't think her smugness is a symptom, I think it is the very source of her power. A Power with deep wells that enables her to take massive physical and psychological damage, yet instantly recover.
Even Sonya recognizes it when she is ambushed by two other assassins - Yasuna could withstand more damage than a trained killer, and that is how she was differentiated from the mortals.
I theorize that her aloof and spontaneous nature is the real symptom which needs discussing. It reminds me of Azatoth, the Idiot God who rules over the universe, who is likely also smug - distant and aloof as galaxies and whole universes are created and destroyed at whim in an instant.
It is truly a mercy that we cannot comprehend the Smug.