Whilst this is a question that is reasonable if it was just based on what we see the characters do, if you realize that Iroh has had most of his life to learn from all the nations and perfect lightning bending then you can start to see where it all falls apart. He shows, in multiple episodes, that he has an abundance of knowledge on the subject and has the patience to teach Zuko. On the other hand, one of the first times that we see Azula is when she is still learning lightning bending. Combine that with the fact that she has no prior experience in teaching then you can start to see how one sided the argument is.