The thing is, a more accurate pistol might extend our range from 20 yards to 50 yards but a decent rifle, even with just our base martial and an afternoon's instruction, will extend it to 100+ yards. If we do take actions learning it, 500+ yard shots are doable. On the rare occasion that a job can only be done by sniping and Mathilde is the only one who can get into position to take the shot, she should bring a rifle. Her difference in skill level between the weapons is overwhelmed by the difference in capabilities of the weapons themselves.
However, a longarm is a primary weapon, as long and bulky as her sword and considerably heavier. Mathilde is not a handgunner or sniper, she's a mage. She has a lot of magic to help her get into close combat with high value targets and more magic to make her effective when she gets there. She carries a firearm as a backup weapon, for when she needs stand-off power in a hurry - that's why she learned pistol rather than handgun in the first place. The question then is, is a marksman pistol an improvement over her revolver for the uses she needs it for? I would say that, although she has done a long (for a pistol) shot assassination in the past, the increased accuracy of the marksman pistol is irrelevant at the ranges she shoots at normally. More relevant is that that thing is fucking hand cannon firing a large calibre, pointy bullet with lots of penetration and stopping power. Against that, it's single shot whereas the revolver has eight shots so if it takes three hits to do the same damage she can do that and then shoot the next opponent. If she misses she has another shot.
I can see it being worth her while having a decent rifle for niche uses but not going overboard because she isn't going to put in the time to get good enough that it's the gun limiting her performance*. I don't see the marksman pistol filling that niche as well nor replacing the revolver as her sidearm. She could carry it in addition but it's a big, heavy pistol that takes different ammunition - better to carry a couple of extra pre-loaded cylinders for the revolver.