A lot of people aren't sympathetic to Aya simply because she exists as a possible love rival to Hikari. The backlash didn't start with her narration dump in chapter 22, it started with her kissing Yuu in chapter 21. You can check the thread, people start complaining about NTR by page 2. If people came here for Yom I'm not sure why they are surprised, the main gag and plot of Ganbare Douki-Chan is Douki-chan getting jealous and fearful of the other girls getting with the male lead before she could. One of the characters in that series is similar to Aya here. She even has a scene with the male lead that's similar to one in this chapter. Maruto Fumiaki is apparently known for these kinds of stories too, but I've never read any of his so I can't say for sure. A lot of people wanted a straightforward childhood friend romance story, and got upset when it wasn't. The story is still not over and, chronologically, we haven't gone past the point they kissed, so it might still end up how they wanted it originally. Well, minus the straightforward part.
Aya's arc starts a year and half before the cliffhanger, both Yuu and Hikari are in their last year of middle school. The high school entrance exams are in February and the results come out not that long after. The new school year starts in April. Hikari says she didn't really start looking at Yuu with romantic interest till about a year prior, which would have been about the summer time of their first year in high school, and Aya's repeat year. Assuming right after this chapter Aya and Yuu start going out, Hikari starts falling for the Yuu that's either been going out with Aya for a few months, or broken up with her recently. Aya might have made friends with Hikari on the heels of breaking up with Yuu. We'll have to see.