Not that I'd be an expert on these either, but I'll give it a shot.
First, ratings and whether we can say they're good or not, really depend on many things: channel (esp. cable or national), timeslot, what else is airing, etc. So it's not like good and bad are set in stone.
But to start on with, you can see the top viewerships
here. As apparent, nowadays we don't really see those monstrous 60%+ ratings anymore, unlike over ten years ago.
You Who Came from the Stars was a huge hit and it barely peaked on around 30%. For bad ratings on a prime time slot - I'll throw
Pretty Boy for an example. They had ratings around 3-4%. That's just bad. Weekend dramas are a thing of their own, gathering those high ratings (expect 20%+).
For cable, the trend in ratings has been opposite: they get much higher ratings now. 1% used to be considered 'good'. But now, with likes of
Reply 1994 (finished with 10%+) getting high numbers, it's pretty apparent it's changing.
Misaeng (another huge hit) averaged around 5%. A low rating then? Just recently,
Ex-Girlfriend Club got episodes axed - no doubt because they didn't even reach that 1%.
