Lady Capulet refers to Paris as "valiant" and handsome. In act 1, scene 3 she tells Juliet that she will, if she studies Paris' face, "find delight writ there with beauty's pen." She also tells Juliet that by marrying Paris she will "share all that he doth possess." Paris happens to be very wealthy, so "all that he doth possess" is actually rather a lot. Juliet's mother is happy for her daughter because her daughter will, by marrying Paris, increase her own wealth and also the wealth of the Capulet family.
Later in the play, at the end of act 3, Lady Capulet tells Juliet that she will marry Paris "next Thursday morn." Lady Capulet is obviously very excited for this marriage to take place, and she calls Paris a "gallant, young and noble gentleman." When Juliet says that she does not want to marry Paris, Lady Capulet calls her a "fool" and, what's more, says that she wishes Juliet "were married to her grave."
Lady Capulet thinks so highly of Paris mostly because of his social status and his considerable wealth. In Jacobean England, the primary reason for marriage among the upper classes was not to confirm the love between two people but rather to cement and improve the financial and social statuses of the two families. According to the standards of the time, therefore, Lady Capulet thinks that her daughter is a "fool" to decline the marriage. When she wishes that her daughter were dead ("married to her grave"), this is because she is so upset at the possibility of losing the social and financial security represented by Paris which she and her family would gain from the marriage.