To all intents and purposes, she adopted that from Lisanna. I wouldn't go to say which of her nature is her "true" nature (demon Mirajaine or "peaceful" Mirajaine). Even without her powers she was readily pissed off at Laxus when he let Natsu steal that S-class quest from the upper floor.Originally Posted by Kraco
If "preferring peaceful methods vs readily wanting to duke it out to solve a problem" was the mentality you're referring to instead, Natsu (mainly) and Gray/Gajeel are pretty much the only other ones who fall into the "duke it out" category. It's a maturity thing, more than anything else. Anybody more senior to them in age gives me the impression that they too would take the peaceful route first.
That's not to say Mira won't fight (and fight hard) when fighting is the only option. She beat Freed to a pulp when Elfman was going to be killed. She Satan-Souled with Markrov and Erza after the 7-year skip against the harassing guild when negotiations broke down. She knocked Jenny out when she was put on the spot in the contest.
The S-class exam argument doesn't make sense. If anything, Mira would take it easier on her teammates than outsiders (she didn't though, and just went at them anyway). It makes no sense to try harder against your own members than oppressive outsiders.
Mira's at least as strong as Gajeel/Natsu/Gray in theory. The writer just doesn't want her in the picture because one strong female member is quite enough to focus on. Her fight on Tenroujima against Asuma was done while she was injured to give Erza the spotlight instead.