So, i get your point, but consider - these youma are strong in an ambush but aren’t necessarily that strong outside of being able to blow people up. Anything that can show up in those numbers, especially in a shounen setting, isn’t usually all that dangerous (to the main cast*).
To your point about mimi, she was busy fighting the old lady who we’ve been told is equally strong if not stronger/more skilled than her, and once she caught back up to mimi the damage was already done.
Furthermore, in future, we can be pretty sure that the antagonists will be doing everything they can to make her limit herself by putting her friends in danger, so, yes, she will probably be actively nerfing herself sometimes and that’s part of why we love her.
And if none of that satisfies, maybe she got some kind of passive buff from being chosen as a “vessel” whatever the hell that means
Tl;Dr: Stop getting mad big cool samurai lady minced those guys like the fodder they were, no one gets mad when the fucking stormtroopers get steamrolled and there’s a reason for that