Her fight against Ulti was more about her standing against unreachable target.
More than fight it was more about character.
Nami saving Zeus was the main reason Ulti get defeated.
Difference between Nami and Ulti was shown to be large. That's why Oda drew the scene of Ulti clashing with Luffy and Yamato to make it more clear.
As Luffy said "you will fall in some way" and that was the same thing shown by Nami.
Nami who can almost die against Ulti , still decided to face her when she hurt Tama was great display of character. There was no point of that scene if Ulti was someone that Nami can defeat easily.
Nami defeating Ulti without outside help could also harm her scene of Nami declaring Luffy will become Pirate king. It's impact Increases more as Ulti is someone Nami can't defeat at this point and Ulti can kill Nami at any point and she still manages to defeat her because she kept fighting and decided to save
Zeus ( the same person who was trying to kill her at the start of raid).
Oda didn't gave any other strawhat power-up so much development. It was literally fours years long. Zeus as a character was better than many wano character. He actually has a it's inner conflict and personality.
Nami helpless was necessary to show the shine of her character. It come across as bad because of both reader and Oda. At this point readers want everyone to look and do badass thing. Everyone should have magical haki and just look cool.
Nami and Zeus combination is better if done well. Actually the best Power-up. Setting aside whole monkey King parallel and it has potential of showing interesting and creative things during fight. But here's come Oda. Oda should not hold back Nami and at recent he is giving too much importance to fight. Maybe readers are starting to affect him and obviously fights are more easy to create hype , no matter how much bland.
Nami vs Ulti or Sanji vs Queen are only two great fights in wano. Actually has everything. From gags to seriousness to character moment. Nami didn't get proper conclusion which Sanji get. That was the only difference.
Sanji arc is in the end and I feel like Nami has more potential of growth now , that's the reason we didn't get proper conclusion, it decreases the hype of future development. Oda likes to do it , just see how was Sanji treated before WCI and Nami isn't even bad. Nami has much better moment than other strawhats. Just like we talk about last time her moments get overshadowed by bad things, most of them aren't Nami fault.
Especially what happened in Wano , something is definitely coming. I don't see heroine getting sidelines in Endgame. Hope I am right , I have too many theory about Nami , let's see.