Doc's the most important guy in Cannery Row. Not only does everyone owe him, but the whole plot revolves around a couple of parties in his honor. That makes him the protagonist, right? Well, we're not so sure. See, he doesn't actually do much, and we're not sure he has any real conflict or change over the course of the novel.
He may be the man of the hour, but he mostly just goes about his marine life collecting business.
Mack's not as important to Cannery Row as Doc is. It's not like anyone really owes him anything and people sure aren't beating down the door to get his advice like they do to Doc.
Yet Mack might also have a shot at the Protagonist medal: he's the main mover of the plot. Yes, the book is about parties for Doc, but it's Mack who takes the reins and plans those parties, and it's Mack who seems to have some sort of revelation at the end of the novel.