Could be, Geekboy. I didn't see any overt unhappiness in Johnny. But I was always struck by his saying, "I'd do anything for my Godfather," and not, "I'd do anything for you (emphasis added). In the novel, in an earlier setting, Tom visits Johnny in Hollywood to tell him that Vito will help him get an Oscar, and will help arrange financing for a movie Johnny wants to produce. Johnny asks if he'll have to do favors in return, and Tom says of course he will. "The Godfather has to ask me himself," Johnny replies. "I won't do it if you or Sonny ask."
If I remember correctly, Tom was impressed by this, and by other comments made by Johnny. He flew into Hollywood thinking Johnny was a flake, but flew out with a new appreciation for why Vito thought so much of him.
