This. We've seen how the writers treat characters they don't want to write for: that's how Iris was the character with the least impact in season 3, when saving her life was the season's goal. That's why Iris became Team Flash leader in season 4 and we didn't find out she quit journalism in the middle of season 4.
They're bad at writing for Caitlin because they're writing for Caitlin at Iris's expense. I know it sounds counter-intuitive but it's the truth. Iris is the second lead, the title character's LI, she's the second most important person in the story and the most important person to the most important person in the story. Getting her right isn't optional.
By disrespecting the dynamics of the show - by making Caitlin, a supporting character more dominant than Iris, it skews the entire story. It's not a coincidence that all the best The Flash episodes (Runaway Dinosaur, Flashtime, Once and Future Flash) are the ones that have not only the best Iris and Iris/Barry writing but puts them at the heart of the story. The Flash will never be as good as it can be because the writers are hellbent on demeaning/diminishing the show's leading lady for Danielle Panabacker and the racist perspective. So paradoxically, that's why the writing for Caitlin will always be bad.
But racism/white supremacy has never been about being "the best that it (a story, a society) can be". The goal isn't to write Caitlin well but to write Iris worse. The writing for Caitlin can be bad, but as long as the writing for Iris is worse, then Caitlin has won.