Joan Durbeyfield might not appear too often, but she's an important character, nonetheless. In terms of the plot, she's the main reason Tess goes to the D'Urbervilles' house at Trantridge in the first place, and we all know how that worked out.
Mrs. Durbeyfield is also important to the novel's theme of time and memory. She is relatively uneducated, collects folk ballads, and still believes in old superstitions. The narrator compares Tess to her mother in an early chapter:
Between the mother, with her fast-perishing lumber of superstitions, folk-lore, dialect, and orally transmitted ballads, and the daughter, with her trained National teachings and Standard knowledge under an infinitely Revised Code, there was a gap of two hundred years as ordinarily understood. When they were together the Jacobean and the Victorian ages were juxtaposed. (3.37)
Mrs. Durbeyfield represents pre-industrial England and all the traditions, "superstitions," and "folk-lore" that go along with that earlier time.