
I think the big issue about their relationship is that they gave Jimmy these flaws and such to deal with but gave absolutely nothing for Kara,it just lacks nuance making her this overly sweet golden retriever.
Like maybe if the story actually explored how naive and inexperienced she was in human culture and interactions and life and that got her into trouble or more characters like critiqued her or was actually a character flaw for her to overcome, it would be more interesting.
Or hell,maybe show her recovering from her trauma and abuse from Brainiac and it caused her to be overly trusting to people or just have her incredibly hesitant to trust others outside of the main 3.
Or maybe show her immaturity and naivety and such as actual character flaws that get her into trouble and have to be overcome and show that Jimmy was right, she is inexperienced.
But then that would be too much for the writers who wanted to punish him for "fumbling Kara."
Also making the ending on their relationship like this when you aren't even sure you're getting a S4 is incredibly poor planning and foresight.