The reason why it was so popular is because of the Ai death scene. Most comments I've seen started to dislike it around the love triangle arc, and I feel around the time the anime got released the reception was negative(from what I saw) I think the hype got renewed due to the anime and the recreation of the most popular scene.
Personally the moment where I realized there was no hope for the manga was when Ruby had to have the mystery spoonfed to her, because that solidified that Aqua was a self insert for me
THAT'S TRUE all the hype is Ai's death, I kept and am reading it to see this trash just burn, many of the main characters are so annoying and bad to me, Aqua and Ruby with age gap, and incest, Akane is getting worse and creepy... her encouraging Ruby with her love with a 30-40 old man, Mem-chan taking picture of Ruby and Aqua and saying how cute and like a couple they look, I like her but when those words came out of her mouth.... I find Kana unlikeable (maybe cuz I hate tsunderes) but I don't hate her, I was so happy when she said that Ruby and Aqua being all over each other is creepy.
1. not warning about the dark topics, like Suicide.
2. the main dude is inlove with his mom
3. Ruby does not care about age gaps nor incest.(nor does Aqua as he is inlove with his mom)
4. and more that I am too tired to get into.
I know it's uncommon to have those in anime/manga but from my friend and how everyone said how good it was just said "oh it's dark" I like dark stories, but the manga just fucking grosses me out. Seeing how big the anime is also, people ignoring the elephant in the room bothers me, it felt like when I was a kid watching "sakura card captor" in sub at like 11 and seeing those age gaps.
It was a kids show, little kids watch that show and think that a teacher and a 8 year old student in love is ok. From how big Oshi no Ko is I feel like we are over hyping it, we are ignoring the weird shit and praising a show with pedo, incest shit, I don't want future big animes/manga with that shit in it