"I didn't smash enough hot young box as like a high schooler so therefore I've missed out on my entire life and nothing I do matters anymore," is such a whackass mindset to have. Literally aspiring to peak in high school. Bro you can work out starting now and I guess you can't prey on high schoolers when you're 25 but trust me that is not a sign your life is over. High school doesn't matter man there's a reason the phrase "peaked in high school" exists. The second issue is that if he wasn't making every moment count before then he isn't going to just because he travelled back in time.
I think people dislike the other characters for essentially making fun of him for trying to change and him for not reacting appropriately to this. "High school debut" I get it. They're all basically shaming him and proving that his "true self" was worthless. If they didn't think that, then they wouldn't be making fun of him for it. He has like the worst friend group I've ever seen, can't even say one nice thing about him improving himself.
And then the gaslighting about his true self and shit from that same girl when that whole "high school debut" thing happened makes it hard to believe it's not her fault he stopped talking to her when she's such a bitch. I get the feeling everyone reading already noticed this which is why they aren't so hot about these characters.
Great art though. Feels like most manga are carried around on their art so I'm surprised it didn't work here because I've seen waaaaay worse stories get waaaay more forgiving treatment.