Exactly. She only appears in one movie.
She only appears in one movie, yet we know more about her than we do Admiral Ackbar. I agree that she doesn’t have a complete character arc, but she’s far more developed in TLJ than Admiral Ackbar ever was in ROTJ. She helps give characterization to one of our main protagonists and she has a very direct impact on story and plot.
So I would argue that she is a character while Ackbar is not.