Piggy seems to be quite a unique character in the novel: he seems to be the only true voice of reason, yet he is often incapable of physical work, shows little leadership skills, and is pretty naive as well.
For his positive qualities, he is the one who voices what should be done on the island to maintain order and maximize their chance of rescue: he suggests moving the signal fire, is loyal to the concept of the conch, etc. He also proves to be a loyal and true friend to Ralph.
Despite his positive qualities, Piggy is not taken seriously and often teased due to his physical appearance and health - even Ralph teases him at times. Piggy might have the best ideas, but he lacks the ability to lead the others and seems rather immature when it comes to social and coping skills. Finally, Piggy seems to be rather naive - he truly believes that Jack attacked his and Ralph's camp simply to get the conch, then confronts Jack for not obeying the rules of it.