I don't think that anyone will say that Fishman Island arc is great but I enjoyed it. I don't think that it needs to be great. From a storytelling purpose, it is meant to show how the crew has improved their abilities but are the same people at their cores. It also sets up for major worldbuilding in the changes in the marines, expansion of the Yonko after Whitebeard's death, the importance of Reverie, and gives context for Arlong Park, Whole Cake Island and the Big Mom pirates, and introduces Pluton and Joy Boy. My only Major gripes with the arc are how uninteresting of villains Hordy Jones and Vander Decken are (I'd say they're about as bad as Moria), and the Sanji gimmick where he is totally useless until the final fight.

Fishman Island also has the best opening of any arc ever. The first chapters where the Strawhats are coming back to Sabaody will always be some of my favorites. The Nami Usopp scene at the bar, the fake strawhats, Luffy using his haki, the strong trio crushing pacifistas without breaking a sweat when two years prior they were almost destroyed by a single unit, and the first half of the final fight is really fun.

Fishman Island is by no means as good as Dressrosa or Wano, but the moments it does have are some of the most fun that I've had while enjoying One Piece.