I don't know why you would count Fishman Island as part of the Dress Rosa Arch, AND
I don't know why people rag on Fishman Island-- it may not be no Ernie's Lobby, but then what is?
Fishman Island did a lot for the series, including:
-Showcase the results of Luffy's training (How is defeating a White Shark drugged up Fishman UNDER THE OCEAN not impressive?)
-Set the tone for the post-skip story, and set it's direction with the interaction with BigMa
-Poseiden
-Jinbe
-Give us an organic knack for the crew's updated personalities
-list goes on
Fishman Island had it's own, unique plot that was quite rich and I still see as a key cog in the story's future development (quite a bit more than you can say for most of Skypea, which itself is still a great arch).
Anyway, shocked thay we've been on this arch for 2.5 years, strongly think there are story bits you can bitch about a lot more than Fishman Island.