The nurse is kind but coarse, loving but ineffectual, both a mother and not really a mother. She gives poor advice and is not a good disciplinarian. While she often provides comic relief, the character of the nurse was meant to be seen as problematic.
As a surrogate mother, her replacement of the biological mother would have been seen as problematic for the majority of Shakespeare's audiences. For one thing, she is of the servant class. Although now we tend to think of the Renaissance elite mothers as constantly going to wet-nurses for the care of their infants, this simply was not the case. Rudolph Bell, who studied Renaissance mothering extensively, said the overriding opinion was that "mom should do it." Back in this era, it was believed that a woman's breast milk also carried with it character traits. The nurse's tendancy towards bawdiness, her lack of education, her poor reasoning all are hints at Juliet's ultimate fate.
As a servant in the Capulet house, the nurse is both a mother-figure and a friend to Juliet. For the audience, she is great comic relief. The nurse bumbles through conversation, digressing from the topic at hand, and saying vulgar things, to which Lady Capulet blushes. Juliet confides in the nurse her love for and marriage to Romeo, and although the nurse supports Juliet, and even helps them meet secretly, she is also rather practical in nature. Ultimately, she tries to persuade Juliet to marry Paris, since he is such a good catch.