Someone's peacemaker is someone else's enemy.
It depends on the eye of the beholder, hm?

To some, Narcissa may have been overly nice, pampering her son and protecting him in so many different ways. She also recognized when to put herself aside, like in DH, that she did not say the truth about Harry being dead, and she does go out of her way to protect him. To Draco, she's nice.

But you think of all those people she's put down, and her opinion of a lower class or caste, of the people who just see her as being married to one very evil, evil man, and they think she's mean.

You can't be one or the other, in my opinion. She's a devoted mother, which is certainly a high plus on her side, but she has glitches she needs to sort out before being considered an utterly nice character. I don't think she's utterly mean either, she's just, like everyone, committed certain lapses in judgement.