Can you find a sharp poster -- maybe an impressionist or something else widely appealing -- in that exact size? I'd just tape it over the mirror for now, inside the frame.

I would layer the area. Essentially ignoring the mirror but placing art work and framed mirrors leaning against it with additional accessories.

I would leave it alone and not draw attention to it. Let the buyer decide how to decorate. I seriously doubt it would prevent someone from buying your house.

it looks blah because you don't have it staged.

do something like this to the mantle and the hearth

looks way more inviting

(not the picture w/words, but the vases are nice, maybe a box of wood or a throw for the hearth

and if you really want to stage this room, take down that ceiling fan and put up a nice chandelier, or a semi flush mount light that reflects back something nice.