Some people blame manga because they think it might promote certain values or ideas they don't agree with. Also, there could be concerns about how it affects time management or study habits for some.
Maybe because they think the managers aren't handling some issues properly or aren't promoting the manga effectively.
Maybe because they have different expectations or don't like the style or plot.
Maybe because they think the colors don't match the intended mood or style of the story.
Maybe because they think kids imitate what they see in cartoons and that can lead to bad behavior.
Maybe because it doesn't meet their expectations or has content they don't like.
Maybe because they have high expectations for the anime to be exactly like the manga. When there are differences, they get disappointed and start blaming.
Some manga might be considered bad if the pacing is off, the dialogue is clunky, or if it fails to deliver on promised themes or genres. Sometimes it's simply a matter of personal taste not aligning with the content.
Maybe it's because the storylines are too predictable or the characters aren't developed well enough for them.
It could be because of poor art, a confusing plot, or unlikable characters.
A manga might be seen as weak if it fails to engage the reader. This could happen if the pacing is off, the world-building is lacking, or if it just doesn't offer anything new or exciting.