"The Yata Mirror was equipped to Itachi's Susanoo, in its left hand and, is considered one of the three sacred treasures. It is said to be endowed with all nature transformations and as such changes its own characteristics to completely negate any opposition."Ahaha you are pointing to feats all of a sudden? Show the feat where the hats mirror wasn't a round shield.
Keyword: change it's characteristics, meaning it has no true form.