Many say Stannis would rather break than bend...like iron...but he's proven one of the most flexible characters in the novels so far
Stannis gave Davos, a baseborn man, a lordship and eventually made him Hand of the King. This was after Stannis sentenced him to death. If Stannis was iron he would've stuck to his words and killed Davos, but instead Davos was saved and elevated to a position of great power. Stannis also takes Davos' advice, as well and Jon's and even Asha's.
He has frequently proven that he is not iron, and something more akin to steel. The characters who never took advice have arguably been broken - Ned, Robb, Theon, even Dany is down a pretty dark path, however the characters who are more flexible, such as Stannis and Jaime, willing to adapt and change their gameplan, are surviving and thriving.
So Stannis isn't iron, he's prove now he is able to bend before he breaks.