TOS is a good show but it is also overrated unless you judge it conditional on its circumstances like tiny budget and so on, then it is of course fantastic.
If a contemporary Trek show focused mainly on three characters and produced as many mediocre and bad episodes as TOS it would be teared to shreds by Trek fandom.
Not to mention the ridiculous continuity obsession of many people who at the same time have the uncanny ability to deny that TOS (rightly so) made stuff up as it went along.
I would argue that if you're looking at TOS seasons 1-2, the ratio of good to mediocre or bad is actually very high, probably higher than any other Trek series' seasons 1-2. There's a drop-off in season 3 of course, but the overall ratio for the show is very good.
YMMV of course.