Why so harsh on Miyako? She's still young and haven't fully developed mentally yet, so she still tends to think in extremes as she doesn't have the knowledge/experience to have the confidence to handle things any other way. Besides, she's also traumatized by the same incident that left MC mentally crippled for the longest time, just in the other extreme. As Masaka observed, Miyako herself didn't even understand she was suffering from that trauma during that confrontation with the ogre.
Thankfully she adores her bro and we can count on him (plus Sen) to rein in her worst tendencies.