Photographer leaning on the mirror ?
(Causing movement on that and not the rest, or even vice versa)
Slight movement of the camera to the left while the person moves to the right (matching the mirror image ).
But honestly : dirty lens on the right side , or a bad lens issue / decenter … maybe a lens with a strong focus curve at the sides .
I remember something like that (looks like motion blur ) on one side of a cheap 80’s Minolta 17-35 f2.8-4. the left side and the center wasn’t bad , but the right side thought me new levels of 'blurry in the corners '.
Don’t think it has anything to do with light travel distance through a mirror and focusing. It doesn’t look like (simple) out-of-focus blur to me.
My guess is on (sharp) fall off in the lens quality at the side , maybe made worse with a bit of motion.