Nami get a power up by Zeus and Usopp got an haki grow in Dressrosa.
The issue here seems to me that you are waiting from Nami and Usopp things that they never show even pre-timeskip. They are not fighters who grow through every fight.
Luffy and Sanji are fighters, that the difference.
And you will see this with the fighters of the crew. You already saw that with Luffy and with Zoro by the way.
From how the fight played out the only difference with pre-TS is that in this fight Nami and Usopp were totally outclass so Oda used Big Mom to help.
And we do know now the limit of Nami and Usopp. We know they are weaker than people on Tobbi Roppo level .
The thing is...there were no build up for Nami and Usopp fights, just your expectations. Which is not the same thing.
And I think it is the big issue of the One Piece community right now, it seems that a lot of people are unable to accept that their expectations were off. But it is also Oda fault, he waited to long to deliver on the fight which made people develop wild expectations + yeah the choice that he made to not take time on fights anymore was clearly a mistake imo seeing how he is handling his storytelling.
I understand the frustration but some criticisms are not legitimate imo.