I agree with you that dumb fight has no sence, but going farther in plot with weak characters have even less sence as well. This is not Naruto or Bleach or Tairy Fail. Those characters are important...you couldn't just leave them be. That is the main problem.
I don't have a problem with Usopp and Nami fighting. It's just that we know Ulti and P1 are fucking strong right?
Making this a gag fight is not benefiting Usopp and Nami in any way in my opinion.
I agree SH's all should gradually grow in power but normally.
Is Tama feeding Ulti and P1 with Dango for Usopp and Nami to troll them with Usopp's plants or Nami's tempo growing in power?
For me personally no.