Honestly, her anger is rarely a loud spectacle, which makes the few moments it does surface hit like a truck. Characters around her are often caught off-guard because they're used to her quiet compliance. When she finally snaps, like during the battle with Doma, you see Tanjiro's immediate shift from encouragement to deep concern—he doesn't scold her, he tries to reach the girl behind the rage. It's less about reacting to the anger itself and more about reacting to the pain causing it. Even someone as perceptive as Shinobu seems to have prepared for it, treating Kanao's bottled emotions as a symptom to be carefully unpacked, not a flaw.
I think that's the core of it. The narrative treats her anger as a sign of her healing, not a regression. The Hashira and the other Demon Slayers don't punish her for it; they guide her through it. Inosuke and Zenitsu don't really get it on a nuanced level, but they accept it as part of her. The reaction is a collective, quiet acknowledgement that she's finally feeling things strongly enough to get mad, which is a big deal for someone who was trained to feel nothing at all.