I liked Mary. She was a total badass. If she was moody at times, well, could you blame her? She was the good child, the one that got excellent grades, always did her chores, was kind and respectful. She had to watch as bratty Laura sucked up all of Pa's attention whenever he was in the room. Laura was constantly mouthing off, skipping school, being late for class, ducking out on chores, getting into trouble, starting fights and getting involved in crazy schemes and what not. Not to mention wishing death on her baby brother. Did Charles ever punish Laura? He was quick to threaten violence against other people's kids.
The worst was when Mary lost out on that dictionary and not soon after, Charles gave one to Laura (who hated it) as a birthday gift. I think that was his way of sticking it to Mary because she wasn't as tomboyish as Laura and he didn't have any use for her.
Anyways, Mary was at her best when she was sticking it to the assorted hicks and rubes that were all over the place back then. Like in "Money Crop" where the guy was like a day or two late with everyone's seed (he got in a wagon wreck) and the town was flipping its collective you-know-what. They all congregated on the steps of the Mercantile, swearing and yelling and driving away all the business and accusing the guy of running a scam, then accusing his wife, then Charles of being in on it. Mary, who was like a 12 year old girl at the time, went out to all those adult men and berated them. Tremendous.
An even better example was "Whisper County", an extremely backwards, anti-technology town run by a religious zealot named Miss Rachel Peel, who is constantly spreading lies and trying to prevent kids from learning anything but the Bible. At the end, Mary finally releases the chokehold this woman has over the community by taking her lies and exposing them in front of the whole congregation, until Rachel Peel can't muster any mumblings about "Fake News".
Laura was such a disruptive child and Carrie was so needy and helpless, I can't imagine either one of them functioning without Mary. Thank goodness she was there to help keep all the younger kids in line. Laura probably would have went on her way to murdering people had it not been for Mary.