This is actually a very interesting plot device by the author. By introducing an overwhelmingly annoying character in Mashiro, Iroha's antics are somewhat downplayed despite both being annoying and false pretenses. Just note, earlier she was trying to disrupt an important talk despite the MC seriously telling her off, potentially destroying the future of the group (which includes her brother and the man she loves supposedly). She also made heartfelt confession and withdrew it as a joke (it got downplayed again but seriously, just imagine it being done to you in real life). Point is, she's equally guilty of being a terrible person and I dare say this: treats the MC as horrible as Mashiro does. I can only wonder what would happen if you introduce a sufficiently amiable female character.