In practice, almost nobody is marking their games as adult for anything other than nudity or porn. Depictions of non-sexual violence and mental illness are simply not marked at all, and sexual violence completely banned from the site.
I agree that Itch should add better content filters, but in practice you are not missing out on anything by turning off adult content.
You will find some threads in forum of devs that complain that they cannot turn off adult content for viewing, because they marked their game that has mature topics as having sensitive content.
So, devs do use this feature. But since there is less than 30k nsfw games overal compared to the 1.7 million other games, that number will be low in comparison, even if all those nsfw games would be the non nudity kind.
