Mary Warren has declining values. She gets caught up in the play acting. She begins to enjoy her role in the court hearings. She feels powerful in that the court is depending upon her actions to find people guilty:
Mary Warren is the Proctors' servant who seems timid and subservient but who finds a powerful role in a kind of people's jury in the courtroom.
When John Proctor puts pressure on Mary to tell the truth, she almost gives in to the pressure:
Proctor convinces her that she must expose Abigail's lies to the court, which she agrees to do.
She begins to tell the truth. Then Abigail's presence causes Mary to return to pretending. Mary is easily swayed to follow Abigail's lead. She does not stick to her values and morals. She gets caught up in the play acting. She lies in court. She accuses innocent people of dealing in witchcraft. She knows she is lying but she does not change her story. She is responsible for John Proctor's arrest:
She becomes hysterical before the court, however, and soon joins Abigail in pretending that mere is evil witchcraft at work. Her behavior in the court contributes, in part, to John Proctor's arrest.
Just a quick answer:
I would describe her values as very volatile. She is a weak character, a juxtaposition to Procter and how till the hour of his death, he refused to "sign himself to lies". (Yet it's ironic how Procter thinks of himself as within the ranks or her - "we shall slide into our pith together"!)
She is a classic example of the effect of hysteria. She will give in to anyone: first Abby, then Procter, then the court. We never really truly know what lies within her - thus, her values never come to light. Unlike characters like Procter, Elizabeth & Hale, when she went through the "crucible" she did not emerge with her strong elements. She crumbled to hysteria.
Her purpose in the play is too highlight the weak values that some of the Salemn coummunity have succumbed to, which has allowed the hysteria to proliferate.
Quite an important character, actually.