Third Season Star Trek tried to be more bold in the storytelling but their budget was slashed, Dorothy Fontana had left, and Gene Roddenberry had gone off sulking after the Network called his bluff on threatening to walk away from the show if the show was put in the "death slot" (10pm Fridays, when no one was watching.) The Network did not like or want any more to do with Gene Roddenberry.
So you have producer Fred Freiberger having to produce bottle shows as there was no money for location shoots (with the exception of The Paradise Syndrome) and with bottle shows they have to be good interpersonal stories which focus on characterization and well-written dialog (no visuals.) Hence why Star Trek's third season took on subjects it hadn't in the previous two seasons and the stories were more mature and more ambitious
The Mark Of Gideon is a classic example of what could have been a good story, certainly the premise was: overpopulation and the pro-life vs pro-abortion argument that was going on at the time. Where it falls down is the execution of the story. With no money to tell the story properly scripts have to be butchered, corners have to be cut, and things have to be changed. This is no fun. Hence we get episodes like The Mark of Gideon, And The Children Shall Lead, The Way To Eden, and maybe one or two others that were good story ideas but subsequently poorly executed. (Then there are the real stinkers like The Lights of Zetar, Spock's Brain, That Which Survives, and The Savage Curtain which failed miserably and were just bad in every way.)
So Star Trek's third season was definitely a more mature Star Trek but the slashed budget meant they were doomed from the start. It was not "Fred Freiberger's fault" as some Star Trek fans want to believe. If anyone was to blame it was Gene Roddenberry. His first mistake for Season 3: not elevating Bob Justman to Producer, instead bringing in an outsider, Fred Freiberger. The talent was sitting righ there in front of his nose. Stupid decision, Gene!