I've looked into this before for arguments with my mother trying to get her to stop saying there's a gay agenda. Her argument was because I enjoy the videogames and sci-fi I wasn't smart enough to understand the truth. The science has actually been done (though I'm sure more still needs to be done).

Research by a scientist named Schwartz in 2016 showed that those predisposed to anti-social behaviour are also naturally pre-disposed to watch more TV. When accounting for genetic traits, previous research into negative affects of TV didn't hold up amongst children and young adults (mentally, physically yes, but we already knew sedentary lifestyle has negative impacts).

A (I Believe Johns Hopkins but I'm going from memory) 20 year data evaluation released in 2021 showed that excessive TV can contribute to cognitive decline over time (they measured a 0.5% reduction amongst the adults they tested). The main scientist I remember saying they didn't account for type of media and did think educational content like documentaries would have a smaller impact though he couldn't guess by how much. His worry wasn't about the average adult making decisions but rather preserving as much mental capacity long term so as to help reduce impacts of dementia.

A 2023 UK study backed up the 2021 and 2016 study. Excess causes increased health risks and a long term small measurable cognitive decline.

Everything I've seen points to, for the most part, the amount being the problem as with everything. Moderation is key and living a diverse life that stimulates you mentally overall is important. Manga or Marvel movies when you want to relax is not going to rot your brain. Only consuming them will, same as with any type of media or genre.