i'm mainly using the game as my frame of reference, i haven't watched her episode in the anime and while i do have to reread pokespe, i remember her being a bit testy at times there.
marnie doesn't seem to get angry very often. she'll get frustrated from time to time, particularly at herself or when scolding team yell, sure, but not angry. she doesn't shout or snap at people or get into physical altercations. she's very level-headed in the face of annoyances, and when met with things that irritate her, she'll usually roll her eyes, maybe make a sarcastic comment, but move on without incident.
this is not because she is a naturally calm person, and it's not how she always was. while she was shy and timid as a child, she had a nasty temper when provoked and sometimes even got into scraps here and there. mostly when faced with her bullies, so she was defending herself, but still. not the type to turn the other cheek to disrespect.
it was only around the time that she set off on her journey to complete the gym challenge that her behavior started to change. see, in spikemuth, people are rowdy. fights right in the middle of the street are far from unheard of. in spikemuth, if you're rude or say something out of line, nobody's interested in playing nice and holding their tongue to preserve the peace. you'll get back exactly what you dish out, sometimes even more so. and spikemuthians have a reputation for being ill-tempered throughout the rest of galar, to a point where it's heavily exaggerated.
it's because of this reputation that marnie takes it upon herself to break out of the stereotype. it's not because she suddenly thinks being reactive is wrong or anything, it's because she understands that she's the only gym challenger from spikemuth this year (and probably the first one to make it that far in some time) and that eyes will be on both her and her hometown. what she says and does will reflect back on spikemuth, whether it be poorly or positively.
she wants to be a "model" spikemuthian, someone that people can look at and say, "hey, she's from spikemuth, but she's so nice and polite! maybe what people say about spikemuthians isn't true." she has good self-discipline, so she's able to keep a handle on her temper and not act out, even when she really wants to. but that doesn't mean she gets her feelings hurt any less often, and there are/will be times where she says "fuck it" and gets visibly upset anyway because everyone has limits, marnie included.
but it's because of this shift in mentality that she's less angry over all. and maybe because of that, her feelings towards rose for what he did to her parents and piers and spikemuth, all of the people she loves and cares about, are just kind of... meh. there's still a pretty big grudge there, and it's probably impossible for her opinion of him to ever change, but she feels like he isn't worthy of her wrath. he's just a sad, greedy man who got what was coming to him, and beyond that? she couldn't care less.
the same goes for her bullies and the people who gave her trouble in her childhood, only to turn around and sing her praises once she started becoming known and giving spikemuth good pr. they're sad people. they're not worth her time. she has so much she wants to do, needs to do, so she's not going to waste a moment of it stewing over past wrongdoings.