There are a bunch of PPSSPP settings in the retroarch-core-options.cfg. Many may already be set to default, but many are left alone, such as anisotropic filtering which is cheap on performance but improves the visuals quite a lot. Just like most other cores, there should be explanations and a list of possible variables for each setting in the Libretro wiki.
Explanations of the core options would indeed be nice. If you like, you can request an editor account there and add them. If not, someone else will get to them at some point.