The narrators of The Pigman, John Conlan and Lorraine Jensen, are two teenagers struggling to understand themselves, to get love and recognition by their parents, and to find a place in their school peer group. They're typical adolescents, in other words. That, and the structure of the novel, is what makes them unreliable as narrators.
An unreliable narrator is a character who tells a story that you're not quite sure is true. They have ulterior motives, or they're influenced by external forces which might not be obvious from a simple reading of the text. In The Pigman, John's trouble-making reputation and Lorraine's abusive relationship with her parents make them suspect, because you should wonder about their motivations. Why do they say the things they do about Mr. Pignati? They pledge to report only facts, but can you trust them? After all, they pledged to themselves to take good care of Mr. Pignati's house after his heart attack, and John convinces Lorraine to have a party. Lorraine goes along because she wants John's affection, even though she senses circumstances getting out of control. That should make you, the reader, wonder about their narratives.
The other big factor in creating unreliable narrators is conflicting perspectives. In this case, you're offered two. John and Lorraine alternate storytelling between chapters. Whose is the point of view closest to the truth? How will we know what really happened, when we only get one point of view per chapter even though we know there are two people telling it?
Unreliable narrators do not necessarily make for bad stories. They're devices chosen by authors to set the tone of their stories, to create uncertainty and feelings of conflict.