This easter egg is only present in the MSX(1) version. At a certain point in the game, if you speak to the King of Ladatorm without having anything equipped with the princess, he'll give you the Skimpy Swimsuit. (And now you know what La-Mulana was referencing.)
The Japanese text translates to "Woah, that lady behind you isn't even wearing cotton clothes! It's pitiful that somebody so cute doesn't have anything to wear."