What is Harper Lee's message about truths and lies in To Kill a Mockingbird?
This is a difficult question, because you are asking to give an author's views from reading her book. Moreover, it is a rich book. So, I am sure that there will be many answers. Here are my thoughts.
First, Lee shows that courage is difficult. Courage is the act of doing something simply because it is right without thought of calculating loss. This is what makes Atticus a courageous man. He knows that he will lose the trial of Tom Robinson, but he commits to help him nevertheless. This comes out in his conversation with Scout. When Scout asks him whether they will win, he says "no." He adds these words:
“Simply because we were licked a hundred years before we started is no reason for us not to try to win,” Atticus said.
Second, Lee seems to suggest that people are blind to racism. Moreover, racism is pervasive. This comes out most clearly in Tom Robinson's trial and the aftermath when the people of Maycomb complain about Hitler and his hatred towards the Jews. The point is that they do not see their own hatred and racism. Atticus call this a disease.
"...I hope and pray I can get Jem and Scout through it without bitterness, and most of all, without catching Maycomb’s usual disease. Why reasonable people go stark raving mad when anything involving a Negro comes up, is something I don’t pretend to understand...