This is a good question. For starters, most people would not see him as the dominant hero figure in the book. Ralph or even Simon fits that role better. That said, Piggy can be viewed as a hero figure to a certain extent, especially if we acknowledge that all heroes have blindspots and flaws.
Without a doubt Piggy has strong moral views. He believes in doing the right thing and is duty bound. He is also rational and intelligent. He also seeks to be neutral when there is conflict. All of these qualities are noble. And if the group followed them, they would be in a better position. The problem is that people do not listen to him and at times mock him.
What is his blindspot or tragic flaw? It is that he is not able to adapt. Piggy makes sense in civilization, but on a wild island with little rules, Piggy is oddly placed and is not able to understand his new social context. As the story progresses, he dies in a freak accident.