More than half become green already, is there really mature content there? doubtfull
Depends how to define mature content, I suppose.
For instance, mature content can include any of the following: sexuality, nudity, strong language, violence and gore, or sensitive ideology or topics. Then you need to considering that kids are "maturing" (not really) at a younger age now. In short, the definition of mature content is pretty well defined. It's just from different people's view, they alter a bit. I mean, if you're saying that there's no mature content in the green titles, then it just mean that you're much more tolerant to "mature contents".
I mean, even an eechi harem light novel can be tagged with mature content which would be tagged along with a novel that has sexuality, for example.