I don't know, I think you guys might be understating what a massive issue race relations are. It's the kind of thing that needs tons of elaboration to really paint the picture.
On Saboady we learned that humans hate fishmen and think they're disgusting, so we learn of the bad blood between them.
On Fishman Island, we immediately learn that this hatred goes both ways, which is also important, because up until then we could assume, but didn't actually know that the bad blood was in both directions.
Fisher Tiger's flashback shows us some backstory on the issue, and that even someone like Fisher Tiger who wanted so badly for fishmen and humans to get along, couldn't accept humans. The hatred runs that deep between these people that even one of the most pro-human advocates of the fishman actually hated them at the core.
Then the Otohime flashback is necessary to give us the hope the Tiger's takes away from us. Otohime's flashback provides us with an actual hope, someone who is truly trying their hardest to bridge the gap. Someone mentioned that Reverie didn't seem like it was that important, but it was essential. Things like this sort of deep hatred between races doesn't fix itself. If the human culture does not accept fishmen, they won't magically start accepting them. The same is true for the other way around. The only way to fix this is to encourage mixing of the cultures and an understanding between the two of them. The other reason the signatures were so essential was because if just Otohime is willing to change things, it's not significant. It's important for all the fishmen, or as many as possible, to be truly willing to alter their opinion about humans, in order for any sort of change to happen.
Fukaboshi's speech serves to give us the motivation of the villains, and more importantly, to show us the result of generations of hate. The current generation of fishmen barely understand why they hate humans, it's so deeply ingrained in their culture that it is understand, without anything to base it off of. It illustrates how pointless the hatred is, and artificial.
Finally, the constant showing of random fishmen talking about how they hate humans like we've seen in the most recent chapter is also important, for the same reasons that the signatures were important. We've seen how the big players feel about the issue, like the princes and Hodi and Jinbei, but at the end of the day something like this hatred between races comes down more to what the entire group of people think as a whole. If the majority of fishmen don't come around to accepting humans, no progress was actually made. In this case we're given constant reminders of how the regular fishmen feel to show the gradual change and acceptance of humans.
tl;dr: It seems overbearing with how often the idea of racism between humans and fishman is perpetuated, but racism is an incredibly deep topic and doing any less wouldn't be doing it justice. Just about every aspect that seems repetitive is actually really important to the story.
The only thing I'll say is the timing of this arc seems less than perfect, because as interesting as this story is, I think just about everyone just wants to see The Strawhats kick some ass in the New World.
) they would not like it.
