When it comes to One Piece, mangaka Eiichiro Oda has laways had a very distinct art style.
And if you think about it, if he used an art style like everybody else, or intentionally make his characters like bishounen or bishoujo, One Piece might not be as successful.
I guess it payed-off, as One Piece is now one of the highest-selling manga on Earth.
His art style is certainly a bit unique even in Japan, as it really doesn’t conform to the usual moe art style popular in the country.