I get why that's frustrating, darker games are hard to find when they're buried under that much explicit stuff. Right now creators just mark a page as adult content or not, and you either have adult content enabled in your account settings or you don't, so there's no separate step for mature but not sexual. Before turning it on, it's worth browsing tags with it off, because a lot of dark games aren't flagged as adult at all.
With the filter off I still see about 20,000 results under Dark (/games/tag-dark) and over 22,000 under Psychological Horror (/games/tag-psychological-horror), and Gore (/games/tag-gore) has a few thousand if you want something rougher. A separate mature tier does sound like a good idea though, hopefully the staff see this one.