This stance gives +2ac and overwrites / strips any armor spell like mage armor (giving you less AC). The description the stance gives when using it would appear that you skin tightens and turns hard like scales. Could this be reworked to become instead of an +2 armor bonus to be +2 natural armor bonus? I think it makes sense for the stance and would fix it and make it useful.
Is this even possible, I suspect it's more of a 'bug fix' than a suggesting. Unless no DM wants to mess with the monk stances.
Don't expect any changes to monk stances anytime soon, I'm afraid.
I think all AC applied on the character itself becomes dodge AC. Which is part of the unintentional reason why old Duelist which had +2 Deflection AC (read Dodge AC) was so good. That is why the stance applies it on the armor and gives in turn then +2 Armor bonus.
It is meant to be a lesser version of mage armor that you can learn and use if need be. So it is working entirely as intended.
It could be restructured to instead provide +2 Natural AC by applying it to an item in the Amulet slot (if I remember correctly) or perhaps the creature skin; and such a suggestion has its appeal. But as Tala has written it is unlikely someone takes a look on monk stances.