Spock's Brain was actually penned by Gene L. Coon who intended the show to be played as a comedy. Unfortunately, the season 3 producer of Classic Trek, Fred Freiberger, didn't quite like comedies and decided to play it straight...which resulted in this catastrophe. In season 3, there were few comedic scenes except at the end of an episode such as when McCoy asks Kirk if he wants to look like Vulcan officer for the rest of his captaincy in 'The Enterprise Incident' or at the end of 'The Tholian Web' when Kirk is puzzled that McCoy and Spock never consulted his emergency recordings--in case he was dead or missing (and the rest of the crew quietly laugh). These scenes usually happened after the tension in an episode was resolved, however.
Freiberger was a 'serious' science fiction producer and even David Gerrold recognised this when he wrote or rather re-wrote'The Cloudminders'--an allegory on social inequality. By the way, in Herb Solow and Robert Justman's 1996 book 'Inside Star Trek: The Real Story', Bob Justman candidly admits that it was he who suggested that Spock speak to and guide Dr.McCoy through the brain operation. Oh well, at least he admitted that this mistake. But no one could counter Star Trek's clear decline in quality as a result of NBC's budget cutbacks in its final season.