Our protagonist here is not portrayed as a completely naieve fool. At the end of each chapter she does either slightly reciprocate or give a disgruntled reaction, completely unlike the unaffected Shoujo protagonist she is made out to be. I cannot suspend my disbelief that someone with these reactions will reset her expectations every chapter. In the first chapter she implied that this has been going on for a while and that it bothers her. Her inner monologue nearly completely avoids ever addressing her relationship with the "rival", instead forcing the focus on her crush and his relation to the rival.
Are you sure she's truly resetting her expectations between every chapter?
Hiyama has been shown to be rather ... extra. She tends to go well beyond what is reasonable, in both her reactions and her attempts to woo Kimura. Even if Kimura understands that Hiyama is, ultimately, harmless and that she only wants to get closer to her, it's still understandable that she'd meet declarations like "Be ready tomorrow. I won't let you get away with this." with some dread. She may not need to worry about getting hurt, but the same cannot be said about Hiyama hurting herself or accidentally embarrassing her. Case in point: Having your desk covered in lavish gifts as a public tribute is pretty embarrassing and having Hiyama spend that kind of money on her isn't good for either of them.
Additionally, it seems that Hiyama is rather loud and forceful in her manner, which can be extremely off putting, even if you're expecting it and you know the other person doesn't mean any harm. I mean, just on the first page of chapter 4, we see Hiyam cornering Kimura, slamming a hand right next to her head, and then shouting in her face. I'd cringe back from that just because of the loud and thing moving rapidly near my eyes and ears.