This is a most interesting statement in light of the times nowadays. In essence, for that time and that society, Tom Robinson's statement that he felt sorry for Mayella was what is today called "politically incorrect." Ironically, it is in reverse to what usually occurs in present-day America.
Of course, Tom's intentions are none other but to be truthful; the idea of his making any claim to be better than Mayella is fallacious and completely out of character. (Notice, also, that he has not said that he pities Mayella; he has stated that he felt sorry--these words differ greatly in connotation and denotation.)
Certainly, Tom's candidness connotes his complete lack of "situation ethics" that are also employed in modern society. These false ethics Tom does not understand as he is completely ingenuous. The only ones like Tom Robinson are the children, who in their innocence, also do not realize that one cannot always say what comes from the heart. The scene in which Tom Robinson gives his testimony, indeed, reinforces the symbolism of his being a mockingbird as the jury commits the sin of effecting his death when they condemn him for his "thinking he is better than a white."