The idea that we meet aliens who were once our gods is awesome! Unfortunately, the the character of Apollo and his culture are not developed beyond a child that needs attention.
If you met someone who claimed to be Apollo, what would you do? Wouldn't you ask questions to determine if his account match your written history? Ask how he reached Earth. Explore what he has done for the last 5000 years. So many possible questions but barely any are asked.
The female historian quickly falls in love with a handsome man and a beautiful dress. That's very weak and cliché. Far better if she became attracted to him because he told great stories about his time on Earth that fascinated her.
Destroying his power source was a boring solution. How did Apollo travel to and rule Earth if his power was tied to this planet? Surely there should have been a spaceship providing the power. Why didn't he find other worlds who would worship him?
I like that in the original series, it is far more common to have Kirk out-think his opponent rather than the technical solutions that dominate later series. Here, he finds a weakness and exploits it by toying with Apollo's emotions. It was ok but felt weak.
An unfulfilling episode.