In fairness, there's some validity to the "why can't I do what Karola won't" response itself; it's not just some stupid zinger that deserves to be brutally taken down with FACTS and LOGIC.
A lot of the Elector Counts are nowhere near as personally scary in close combat as Frederick or even Magnus are. A natural preference for avoiding violence and gore does not mean that a person is weak or stupid. If Karola has a lot of family she can trust to be capable warriors and agents on her behalf in situations likely to involve violence, then on some level... why isn't that good enough?
Maybe she'll die gallantly in a situation where someone more battle-hardened might have survived. Maybe something will go wrong. But things have gone wrong before, many times in the history of the Empire. And who knows, maybe she'll make correct choices because she isn't personally distracted by that urge to get stuck in and lead her troops from the front. Who can say?
Maybe the fact that Ori wants to help Karola and cover for the areas Karola neither wishes nor thrives in isn't entirely a bad thing, and maybe we don't need to stamp every potential future Elector Count of the dynasty into carbon copies of the same person, the way Magnus in practical effect governs like a (slightly) lesser version of his father.
...
...
...Beyond the strictures of "Don't submit to evil powers", "Don't be a destructive manager", and a third stricture I'll get to in a moment, I could not care less what kind of Elector Karola ends up being. If she goes her entire reign without ever stepping foot on a battlefield, I literally do not care so long as she doesn't inflict unnecessary damage on the people she's responsible for.
Which brings us to the third principle;
whatever you do not, make sure it's because you won't, not because you can't. (1:18 time stamp - don't be Satine)
If Karola's matter of course is that champions fight in her stead because she doesn't want to pick up a weapon and throw down, good. That's the fucking point of bodyguards and proxies, VIPs should not be risking themselves in deadly situations, and while I respect Freddy for stepping in when shit hits the fan I agree with Natasha that he does it more than he probably should.
But if Karola's matter of course is that champions fight in her stead because she's mentally incapable of combat and starts crying and begging and pleading and dissembling to avoid...whatever it is she's afraid of, that's a problem in a peasant or burgher and unacceptable in a ruler.
The former "won't" is a matter of moral integrity, and can be respectable as a sign of intelligence, courage, compassion, and/or caution as appropriate.
The latter "can't" is a character flaw that will be exploited and used to cause injury and suffering to others.
The "won't vs can't" rule and how to manipulate it to your advantage is something that makes an appearance on a weekly, sometimes daily basis in certain professions, and I've seen a fair few "can'ts" ruined by it. It will be much worse in a setting where gods, daemons, and all sorts of nasty magical shit is an ever-present threat.
A pacifist Karola will probably be a great ruler for Ostland. A cowardly, and especially a fearfully dishonest and manipulative, Karola will be dead weight.
EDIT: removed the first part because it doesn't add anything worthwhile to the conversation.