"A Simple Heart" must refer to Félicité's heart. She's simple in that her love for her family and Madame Aubain's family is one thousand percent. She doesn't hold back, isn't complicated, doesn't expect anything in return. Simple can also refer to intellectual simplicity and Félicité, who never had any formal education, also falls into this category. The title is honoring her, because even though she is the least educated, she's the one who understands love.