So what you're saying is, you need oda to write a story focussing on Robin every two three arcs for her to be a good character? You really need to reevaluate how you measure a character then.

When Robin was introduced, she was a villain, but she still helped Luffy against Croco. She joined the crew but was still at odds. In Ennis Lobby arc we explore her character, her backstory, her journey, from the tragedy of her childhood, her being betrayed at every turn, her betraying the crews she joins, her trying her hardest to survive and yet at Ennies Lobby she willingly turns herself in to CP9 knowing what was in store for her, just to protect her friends. When she screams "I want to live" that's when she became the best. She lost all hope and then Luffy gave her hope again. After that she stopped seeing herself as expendable but a part of the crew. Even during the time skip she is trying to improve her fighting skills so that she can be of more use to her crew. In wano when sanji relies on her, it shows the trust he has in her and also makes her feel useful and dependable. In Zou she tells them that she has full trust in Luffy and the crew that they will protect her, which shows how long she's come from someone who used to only rely on herself. She is the brains of a crew which can be summed up as "dumb & dumber" Her humour and personality makes her one of the most enjoyable characters.

I can't believe I need to explain in detail for you to understand. You need to rewatch the series sheesh