Answer:
She thinks that Juliet is using Romeo to stand for the person she would be least likely to marry.
Step-by-step explanation:
Juliet doesn't want to marry Paris. So she, as a Capulet, uses Romeo as a symbol of someone hateful (because he is a Montague) to make her mother understand that the idea of marrying Paris is so horrible to her that she'd rather be married to a Montague. The audience, though, knows that she secretly loves Romeo.