Nope. Well over half of the courtroom-scene dialogue is taken verbatim from the Scopes trial transcripts. In both the movie and the original Lawrence and Lee play. Authors can rarely think of material that good.
Yes, the characters are fictionalized. But not by much, and then mostly by name. Fredric March was a dead ringer in appearance and bombast for Brady/Bryan, Spencer Tracy evoked the rumpled spare manner (if you subtract 30 pounds) of Drummond/Darrow, and Gene Kelly absolutely nailed (if you add 30 pounds) the sardonic Hornbeck/Mencken.
"He got lost because he looked for his God too high up and too far away." Drummond's words about Brady apply in full to most of the Christians I know.