Or the people who say that are obsessed with subtlety and want everything to conform to the Western standards for art due to cultural hegemony brainworms? Sometimes I feel very guilty for engaging with this sort of media instead of something more "serious"
The majority of popular anime targets young people and is designed to be accessible to people with a pretty low level of media literacy. This is equally true for the majority of popular western media as well though. There is plenty of popular anime that is deeper and more mature than any Marvel movie.
This is compounded by the fact that anime targeting adults, with more serious themes, is far less likely to be made available to English speaking audiences. It inherently has less mass market appeal and there is also a self-reinforcing cycle where anime for kids and teens is more likely to be translated because of western prejudice against mature cartoons and so the western audience for anime becomes dominated by the people who want to watch that type of media giving companies no reason to print and market mature stories in western countries.
Translating media also removes a lot of subtlety by necessity, things that a Japanese person would intuitively understand via cultural context and indirect symbols must be made explicit or else be misunderstood by audiences without that context.
The only significant difference is the cultural acceptance of manga combined with the low production cost of the medium means that when an idea gets popular publishers can iterate on it extremely cheaply and quickly, resulting in masses of very similar stories that basically triangulate upon the aspects of a formula that drive sales the hardest, which is how you get the current isekai trend. Again this is hardly any different from "airport thriller" or "bodice ripper" paperbacks in western culture though.
As far as whether you should or shouldn't consume "immature" media, media serves many purposes, people of all ages, races, classes and cultural backgrounds consume light entertainment. Most people mostly consume light entertainment. I think there is value in expanding your horizons and consuming media that challenges you sometimes but there is obviously nothing wrong with watching things that make you happy for no other reason than that they make you happy.