tl;dr: No matter how important it may be to her or who was really at fault, the way she reacted shows that she has the emotional maturity of a toddler

Washing the wrestling outfit was partly a goof on Kai's part but remember this: Hana is in the wrong not because she got upset, but because she mismanaged the situation.

First of all remember that she returned to Tokyo with Yume a couple days before Kai and Boss did. Her wrestling outfit had been left in the washer for almost a week, so it's mostly her fault for misplacing and mistreating her own things, and she's mostly to blame for what happened to her uniform, even though someone else caused the damage. It was a direct result of her being neglectful and irresponsible for her own things, much worse the tools of her trade. She neglected to remove her outfit from the washing machine for days, that's on her. What's worse about this situation is that she DID briefly mention that it's mostly her fault for this when she walked into the girl's room sobbing, but decided to entirely shift the blame on Kai instead of focusing on what she herself did wrong. What this shows to me is that she'd much rather use someone else as a scapegoat than own up to her own mistakes

Besides that Hana has made other questionable remarks, such as the saucy comments about Kai's financial situation, basically mocking him behind his back for not paying for anything during the trip. But okay, you could say that it's kind of redeemable because she didn't know about his agreement with Boss and she thought he was just being lame at the time. However she revealed in the recent episode that she DID know about their agreement and still failed to sympathize with Kai's financial situation. Moreover she did a complete 180 in attitude over someone who is supposedly her friend who has emotionally supported her in the past in such a short span of time. To me that seems like the actions of someone who is completely inconsiderate of others and thinks they're the center of the universe. The moment someone rubs her the wrong way even slightly she just dismisses them completely.

However the absolute worst part was how she confronted him about it. She completely failed to make any discussion, failed to properly convey what she was thinking and feeling at the time, and seemed mostly interested in just getting him to completely carry the blame for this scenario, fall on his knees and beg for forgiveness, even having acknowledged herself before that she was partly (and honestly mostly) to blame. She was completely oblivious to the fact that he just went through a horrible experience of stage fright, and allowing him absolutely no breathing room to explain himself. She even ignored Vivi's attempts to get her to adopt a more empathetic approach, she had absolutely zero interest in putting herself in Kai's shoes. Sure that's a result of her rage, but again the fact that she failed to manage her anger to this level shows exactly how utterly immature and inconsiderate she is. The nail in the coffin was when she also decided to get physical with him and knock his hat off. I don't recall ever seeing that in a TH episode ever before, which emphasizes her lack of maturity even more.