"Plato's Stepchildren" is just embarrassing to watch. Now I know some will say, that's the point of the episode, but it's supposed to be embarrassing for the characters,
not the audience.
I haven't watched the episode in years and don't plan to in the future. It's not an episode I would take home to meet my wife, if you get my drift.
Sure, there are "funny" moments in nearly every TOS episode...even in "Patterns of Force", which was banned in a certain country for years. I love the moment when Kirk and Spock are dressing up in their "German" uniforms and Spock says to Kirk "You should make a convincing Nazi." Shatner's expression is priceless.
My pet peeve with the whole second season is that they tried to end almost every episode with a gag. Sometimes that's appropriate, usually not.