I think when people say "too silly", they mean the script's posture toward itself and the interest the writers have in constructing the story as a whole. "A Piece of the Action" is a comedy with a fantastical premise, but allows the situation to create real stakes for the characters, and the laughs arise naturally from that. "Shore Leave" is surreal but also a mystery with some tense sequences and a narrative payoff.
TAS is much the same - "The Ambergris Element" is about turning into mermaids, but it's also about first contact with a lost civilization. "The Magicks of Megas-Tu" is about Kirk getting into a magic duel with a Witchfinder in an alternate dimension, but it's also about the ultimate test of Kirk's morals. "The Lorelei Signal" is about the male crew getting trapped in a big vase by all-female energy vampires, but it's also an effective adventure story which results in a moment of heroism for Uhura and the revelation of a sympathetic motivation for the antagonists.
This, meanwhile, was a series of disconnected scenes, such as Chapel screaming at old women in a bingo hall before La'an ran in with a group of thugs who started beating them up. A planet with a psychedelic atmosphere isn't the silly part people object to, nor the fact that the crew get drunk and stumble into shit; the thing people aren't liking, I think, is that the script isn't interested in anything beyond very obvious, very self-insistently "zany" scenes. The bomb plot is the killer because it's the climax of the episode and yet the script doesn't even care enough to ever show the culprit on screen, nor to build up the threat before it suddenly appears.