There are actually only two episodes across all of Star Trek that have been rated below a 5 and that's Sub Rosa and Shades of Gray, both from TNG. Despite everything, no Discovery episode ever made it that low, or TAS episode for that matter. Magicks of Megas-tu got closest though!
Out of the movies, only Section 31 fell to those depths, getting a rating of 3.8.
Though the average IMDb rating is 7.0, meaning that 40% of Star Trek is below average. The most perfectly average episodes include TOS: Gamesters of Triskelion, TNG: Galaxy's Child, DS9: Babel, Voy: Learning Curve and Ent: Fight or Flight.