What is a song or poem that represents by its words the powerful theme in To Kill a Mockingbird?
This is not officially a poem, however it is poetic in its intent. A Letter From The Burmingham Jail, written by Dr. Martin Luther King Jr. expresses the quiet rage, distress, and most of all the strength of the human spirit, all of which filled the spirit of Tom Robinson. Dr. King wrote the letter in the Birmingham jail as a testament to his non-violent philosophy.
You might be interested in Langston Hughes who was a African American poet that wrote a number of poems directed at the plight of African Americans in the United States. You will find a tremendous amount of themes related to To Kill A Mockingbird.
After the trial and the verdict is given, Jem's reaction has always reminded me of the REM song "Everybody Hurts." If you listen to it, you will be able to figure out the theme that is demonstrated. Happy listening.