The Nurse is both good and bad; mainly, she is a comic figure (like Mercutio) in the play, a comic foil to Juliet, in fact. To a lesser extent, the Nurse is a foil for Lady Capulet and a go-between, like Friar Lawrence. To say she is a good person or a bad person is not Shakespeare's intent with her character necessarily. After all, this is tragedy, not a morality play. The plot and characters are streamlined for effect.
Her long monologue in Act I is filled with low comedy. After all, she only has four teeth:
The Nurse is more of a mother to Juliet than her own mother, Lady Capulet. Since the death of her own daughter, Susan, the nurse has raised Juliet. In fact, the nurse knows Juliet's age, and Lady Capulet does not. She remembers it like it was yesterday:
And since that time it is eleven years;
For then she could stand alone; nay, by the rood,
She could have run and waddled all about;
For even the day before, she broke her brow:
And then my husband--God be with his soul!
Later, the Nurse will be a proxy for Juliet in brokering the secret marriage with Romeo. Even though she agrees with Lady Capulet in wanting Juliet to marry Paris, the Nurse nonetheless wants Juliet to be happy in her wishes to marry Romeo. Later, however, the nurse will betray Juliet's trust after Tybalt is killed. As the Enotes editor says,
The nurse's affection for Juliet remains constant throughout the play, even if her advice is of questionable value. Juliet trusts the nurse enough to send her to Romeo the morning after the balcony scene to learn what Romeo's intentions are.
The nurse has brought up Juliet. Juliet emerges from her governence as pure as crystal. When Juliet is lifted into womanhood by her love, and gains thereby moral power and spiritual passion, she sees the conscienceless character of this old woman. When her nurse advises her to marry Paris now that Romeo is banished, she flings the old wretch out of her heart :- " Ancient damnation ! O most wicked fiend !// Thou and my bosom henceforth shall be twain."
Yet the old woman is quite endurable. Proper persons cannot approve of her, but it would be difficult for them to be angry with her. She is too human for that. Her garrulity has its charms, and so has her self-sufficiency. Her immorality is natural and primeval, and , when it is heartless, is more the result of the society in which she has lived than of her original nature. She has good humour, devotion to her house, physical fondness for her nursling, and great respect for the Church.