Scout claims that "dill could tell the biggest ones " (lies) she ever heard. why might dill have told such lies?

Dill was lonely. He tells lies because he wants other people to like him and want to spend time with him. The lies he tells are designed to make himself sound interesting, so people will want to be with him. Possibly, he can't face the truth of his own life, either. He bounces around from family member to family member... like no one really wants him around.