Yet she was able to take out an entire regiment of soldiers after suffering serious injuries and losing a pint+ of blood -- in other words perform super-heroic feats of strength.
You just described just about every action TV show or movie ever.
I *do* know what you mean though. They did present it in a way that made it stand out in a bad way a couple times. I think the issue is they show Mizu get hurt to the point where there needs to be serious recovery afterward, but in the moment the injury barely gimped Mizu at all.
So in the assault on the castle when Mizu gets skewered in the leg (ankle?) really early on, it was a bit of a groaner moment for me. Mizu vs. Everyone was a tough proposition to begin with, but now the hero's leg is severely injured and plunging it in an ice bath for 8 seconds isn't gonna cut it. If you're going to do a big injury followed by an episode full of recovery, save it until AFTER the hero does a bunch of ridiculous shit that makes it look like there's no injury at all.
So if Mizu is going to limp from set piece to set piece, the injury should persist throughout the action sequences, too.
Die Hard is one of the best examples of this kind of thing done right. As the movie progresses he gets more and more injured. He drags himself into the final confrontation with his feet cut to shit, a big limp from the falls, and he's exhausted and covered in his blood, their blood, soot, sweat, and the grimy remnants of a pretty shitty evening. The final encounter is brief and plausible. He exits the building and has to be saved by Urkel's neighbor, then his wife punches the reporter.
This was a guy that went through some shit, and it's a huge part of why we love it. It's relatable. It's why we love Raiders of the Lost Ark (where he spends half the movie running for his life, scared of snakes, shooting the swordsman instead of fighting, etc.) and hate Kingdom of The Crystal Skull (where he survives a nuclear blast in a refrigerator). It's why Jackie Chan became a global superstar.
And if your human heroes are going to be superheroes, lean into it to let the audience in on the fun. Not doing that is why some of the Fast/Furious and Mission Impossible movies sucked. Eventually catching on and playing along is why the rest of them range from pretty good to great (one Fast movie and most of the MI franchise after the first couple). Consistency is key, and Blue Eye Samurai fumbled it some.
Despite the novel (sorry), I do think it's ultimately a minor nitpick in this case. I loved the show. I'm not an anime guy (though I have seen most of Miyazaki), so if you guys have recs for other things with animation of that quality (preferably in English), I'm all ears. It was really beautifully done.