I think this part about Wano citizens being narrow minded is an attemp from Oda to show what centuries long isolation can lead to. We saw a similar development with the Shandians , who were extremely hostile towards any outsiders amd had turned their culture into some very weird form , with all the human sacrifices and fear of the unknown.
Wano's narrowmindedness is also a callback to Japan's past isolation and the still persisting xenophobia in modern-day Japan.
The fishman - human relationship and it's execution by Oda is great as well.
One Piece has this overarching theme of people inheriting the will of previous generations. In One Piece, the individual is not shown only as their 'own person' but also as a result of all the generations that came before them.
With Otohime and Fisher Tiger Oda infact portrayed two different possible ways society may go: Otohime stands for a peaceful attempt, rather refusing to respond with violence even if she gets hurt in the immediate situation. Otohime wanted political change though non violent protests.
Fisher Tiger chose the path of violence and fighting.
Through these two characters, Oda also shows the impact that personal experience may have on an individual. This becomes apparent in the scene where Fisher Tiger says that he can never love humans, because what they did to him was too horrible. Otohime had no such experiences so, for her, it was easier to reject violence as a potential solution.
The character arcs of Fisher Tiger, Otohime and Fishman island as a whole, also further highlight the importance future generations have.
A person who is born today has not personally experienced the atrocities of the past. They might know about what happened , or they might not. But since the events of tye past are not their personal trauma , new generations are able to have a freer look on the conflict itself.
Some might still chose to act violently tho, or , such as Arlong , turn the discrimination of the past around and now become oppressors themselves.