Why do protagonist usually mispronounce or forget the name of opponent or others?
You only have a limited amount of mental capacity for stuff, right? So what do you do when you've got too much to remember? You forget the stuff that's less important.

The joke in having a character regularly forget another character's name is usually a sign that they don't consider that character important enough to remember. It's also a huge sign of disrespect, even moreso when it happens enough to become a running joke. And then, when they do remember, it's a sign that they finally consider that character important enough to make the effort.