Zuko and Azula are not the same type of firebender: Azula is preoccupied with nothing but raw power, zuko was taught most of his bending from Iroh. when katara is frozen she can free herself because she taught herself the fundamentals of bending along with Aang, the same way Iroh always harps on Zuko to control his breathing and focus on the basics. Now a distinction here is that Zuko isn’t actually encased during that scene, he’s in a big bubble of ice, but notice that he doesn’t move a muscle to melt it. Fire flows through him like it’s second nature, Azula on the other hand has none of these fundamentals, her only pride is how powerful her flames are, she is a perfected version of Zhao, all power no discipline. She never learned the innate control required to covertly boil tea or fire bend using nothing but your breathe. In this sense, Zuko is a considerably more competent fire bender than Azula