what does Joe's symbolic release of the mule represent?

There is a strong parallel between the mule and Janie. Recall that Nanny warns Janie in the first chapter that the "nigger woman is de mule uh de world." Janie is the first person to be angered by the porchsitters' baiting of the mule; she identifies with the mule's struggle. We can interpret Joe's "saving" of the mule to his saving of "Janie". In the end, Joe will end up exploiting both.

a mule is a cross-bred between a donkey and a horse and is unable to bare off spring. janie in the story never had any kids.