I've read the whole series over the past couple of days, and I just don't get it. People talk about this like it's a masterpiece and I can't see why. The only real positives I can give it are the reasoning behind the zombies, as it is a rather original take which makes the horror elements and worldbuilding a bit more interesting, and the art being utterly horrifying for the zombies at times making them feel like a genuine threat. I would compare it to the titans in Attack on Titan, another horror series from the same time. Generally humanoid but they're always just a bit... wrong. Odd anatomy, proportions, and moving in generally inhuman ways. Concept and scares can't carry you for 265 chapters though, you need some actual meat.
The characters are utterly abysmal. No one feels believable, despite supposedly having a diverse cast of people everyone is pervaded by a general 2ch feel. Incels abound and literally everyone calls zombies a ZQN (wordplay on DQN, 2ch-speak for a dumbass) even those who show no signs of interaction with the site and have only been a couple days into the apocalypse. The incel aura really does exist throughout every facet of this manga, with people constantly bringing up how it shouldn't be mens job to go into dangerous situations just because they're men (even when they're literally the only choice) and women are treated as either objects to be won over or complete trash. This mentality is also rewarded with women basically throwing themselves at the protagonist once he has done a few things for them. (Oda Tsugumi had so much potential and then was relegated to "love interest" and "plot forwarding device" )
The worst of all is the main character. It starts out very strong -- he is socially maladjusted, frequently talking to himself and generally having unhealthy attitudes to interacting with other people, even those close to him. He is also schizophrenic or similar as he has multi-faceted hallucinations which frequently play into his struggles to socialize. Great! Genuinely appealing for a start, you have a guy who is not a good person and is in a bad place mentally, and the series has you set up to watch him learn through the apocalypse and become a better person with a better grip on himself. To become the titular hero, go from simply saying it to ground himself to actually embodying it. Except... he just doesn't. He never really becomes a hero. He only helps people when he sees benefit in it for himself or sees something he likes in them (boobs). His hallucinations also just kinda... stop existing? Yajima never appears again and they're never addressed in the plot. They just stop once the apocalypse kicks off, barring a single scene partway through that never explains why. His mentality never really gets better out of trying to adjust himself, he just has less reasons to hate people once the only people around him are women who wish to have sex with him. The only place he really improves is in his self-confidence, but being confident alone doesn't make you a good person.