No, I don't think Gene Roddenberry was a terrible writer, necessarily.
I think he was servicable. I think he knew his comfort zone, with regards to what worked for him and he stuck to that. As a result, his talent and abilities didn't blossom as they should've. He didn't strive to improve his craft, because he didn't want to play a longshot with his career, even if that meant he could win big. He wanted a sure thing. That means not pushing anything. Bracket your work and stay within those boundaries so you don't get too close for comfort with subject matter, or dialogue. Don't offend or confuse your audience and you might just keep them ... and your job.
I wish STAR TREK could've been much more gritty and realistic, instead of always being so stylized, but ... it also puts you in the frame of mind, too, where you understand that this is not meant to represent reality. So you look for the metaphors and the subtext to connect with that particular episode's nugget of truth:
Power Corrupts. Beauty and Goodness Aren't Mutually Exclusive. Revenge is Cheap and Unsatisfying. Nobody Wants War. Whatever the message, that is what Gene Roddenberry's STAR TREK was about, rather than taking a literal stance on it as being a simulated map of Humanity's future.