This looks like a good occasion to ask: how reliable is fimfiction.net rating system?

I have cautiously read a couple of top-rated stories on fimfiction.net and found them really good. But I know there are nastier ones out there, and those I would like to avoid.

Am I safe just by not clicking the prominent "View Mature" checkbox? But should't the story automatically fit into "Mature" if it's labeled Dark or Tragedy?

At FimFiction, when an author labels a story 'mature', they have the option of selecting from one of two sub-categories: sex and gore. A story can be Dark or Tragic without those elements, and it's at least plausible that even a story with gore might not be Tragic, and might even be a comedy, ala Shaun of the Dead. So, avoiding the Mature checkbox will certainly help you avoid such stories - but there are some edge cases which might slip through.

Another list of high-rated stories is Equestria Daily's Star 6 ones. I don't recall any that made it to that ranking that include content that's erotic, though some are somewhat dark, such as "Background Pony"'s horror aspects.