Why did One Piece say Luffy destroyed Mermaid Island?

Because the fisherman's prophet saw a figure wearing a straw hat in his prophecy.

Li Xia is a fisherman's prophet. He can foresee what will happen in the future. Through language, she saw that "the man in the straw hat" destroyed Fisherman's Island.

In One Piece Comics 6 10, the straw hat pirates just left Bubble Island, and the next stop is Mermaid Island. After the straw hat pirates boarded Mermaid Island, Li Xia suddenly hugged her head and asked them to drive Luffy away with straw hats. Because Luffy will bring destruction to this country.

The reason why Luffy may destroy the island.

It is said that Luffy's destruction of Mermaid Island must have a purpose, and the most important thing for Luffy is his partner. In fact, Luffy once said in a cartoon that he would destroy the whole island.

Luffy told the zombie in the article "Tripod of Terror in the Moonlight" that if his companion had any mistakes, he would fly the whole island.

Luffy's declaration also means that if Luffy's companions are in danger, Luffy will make a move to destroy the whole island, and Luffy's straightforward personality is absolutely capable.