Are you playing the dev version? As of the current dev version certain pieces of clothing (Right now the list is: American flags, blankets, sleeping bags, house coats, snuggies, and cloaks) can be put on regardless of current encumbrance or mutations. It was added in a while ago to allow crazily mutated players to still have a way to keep warm.
As for the helmet, the reason you can’t wear it is because curled horns prevent you from wearing any headgear.
