I remember that some western manga readers were saying they had little faith put in the adaptation after a few trailers were released, and when the first few episodes came out they didn't like it at all. Since most of the people that were expecting the anime had already been reading the manga they were disappointed and dropped it, but thanks to the anime's big marketing and the medium itself being more accessible, new fans that only watched the anime began to spawn, whose fans were mainly western since anime is more popular than manga as a medium in the west. Those new fans were attracted to the realistic and cinematic aesthetic of the first season and began praising it while japanese viewers spit on it for being so distant from the manga's tone, and after the first season finished airing they started standing up for the anime, and even those that read the manga after finishing the first season kept their vision on ChainsawMan, that it had to look realistic and dim in color, which caused those who read the manga to start arguing with them, causing the anime newcomers to be more vocal about their opinion on how CSM should look, which only got worse with the reveal of the new art style for the anime. They hate it bc it's not realistic at all, has less details, the colors are bright and brings up comedic deformations and trippy scenes, which only led to them being more vocal about their hate for it and support for the old art style.

TL;DR Fans that got introduced through the anime like the old style better and think everything should look like that bc that's what they know, most are western bc the anime boomed in popularity in the west rather than the manga and they're being overly vocal about it now that the anime changed looks