Modders can turn around an item like the clown's hat so quickly because it's literally just a checkbox in the item metadata to turn it on or off for viera/hroth, plus another checkbox to hide viera ears or not.

The main issue has always been that Square never made hatted versions of the base viera hair textures, so they all clip horribly with every hat that doesn't hide hair. As to why viera can put on the lvl 83 dungeon hats and the RPR artifact hat, in case you might be wondering. (Something some industrious modders have also created/added)