In what sense are Mitty and his wife stereotypical characters?
Mitty and his wife are literary archetypes. That means they each fall into a category that we repeatedly see in characters throughout time.
Mitty could obviously be classified as a Dreamer. Each day dream, whether he is a famous surgeon, driver, lawyer, or man about to be shot by the firing squad demonstrates his ability to leave reality. Many visionary characters would fit this category with Mitty. Unfortunately, this ability to dream leaves him powerless to do much of anything in his own reality.
His wife, on the other hand, is the typical nag. She is controlling and always has a reason to complain or try to get him to do something other than what he wants to do. His character is stifled by the control she chooses to inflict on him. This character is regularly seen throughout literature and the best example I can think of is Mrs. Joe in Great Expectations.