Mary is playful, gracious and witty when we have the opportunity to hear her speak. But instead of giving us the scene and the dialogue, and letting us join the spectators lulled by the spell of that most magical of instruments, our narrator callously describes these little conversations indirectly, leaving us with the sense that Mary is glib and insincere, even though we know her to be quite amusing, and only too honest in her opinions for her own credit.

We are allowed only a little glimpse of her at her very best "pretty, lively, with a harp as elegant as herself", innocently attracted to Integrity not Money (just as, in ch5 she was puzzled but approving of Fanny's demeanor("one does not like to see a girl of eighteen or nineteen so immediately up to everything) without quite knowing why, oblivious and indifferent to the effect her cordial opinion of Fanny would have on Edmund.) And then, just when we are getting a glimpse of the best impulses of her heart, the narrator cuts it short and shows us instead the Grants at their sewing and eating. Its as if the ON narrator wants to say "oh, yada, yada, yada, everything nurtures what is strong already, I can't be bothered repeating the rubbish that lovers say."
We are reminded instead that Fanny, suffering at home, has not found Miss Crawford or her brother to improve on greater acquaintance.
Elizabeth Bennet can put her own opinions forward, can express her reservations on the wisdom of Charlotte's marriage while a guest of the Collins, can evade the personal questions of Lady Catherine, can ask the price of a second son, and if pressed, can even make it clear that she dislikes a man for his arrogance, conceit, selfish disdain and ungentlemanly manners, without ever being suspected of being tasteless, unprincipled, or immodest.
Mary is portrayed in a harsher light, so even though we really have nothing worse to accuse her of, than that she is quite unintentionally causing Fanny anguish by unintentionally falling in love with Edmund, and some doubt that Mansfield is curing her justifiably cynical attitude towards men and marriage.
It is interesting also to contrast Mary with Maria. We have not heard a kind word or any proper attention to Fanny in the first eight chapters and years of their aquaintence. She starts by demanding admiration for her music, then neglecting her little cousin, then dobbing on her, full of ignorant pride in her own education (that she intends to give up on her seventeenth birthday),her mercenary engagement, her rivalry of Julia and her callous and conceited decision on meeting Mr Crawford for the third time, when he had begun with no object but of making them like him that if he should fall in love with her instead of Julia, Mr Crawford must take care of himself.
And while we are being distracted by Miss Crawford's improper reflections on her uncle, Maria is lying and scheming so she can show off the wealth that her beauty has won her, to Mr Crawford - how proper is that? And where did Maria learn her cold and cynical approach to marriage?
I can't say that Maria is portrayed more sympathetically than Mary - in fact, I would say rather less, but still, her behavior seems to escapes criticism. Is it the virtue of being a Bertram, and unable to marry Edmund? Or maybe the ambiguous and unrevealed quality of Mary Crawford's principles keeps us focused on her character and allows us to gloss over the known and positive vicious propensities we see in Maria, without comment.