One Piece is not mediocre.
Very far from it, One Piece is one of mankind's greatest achievements is fiction.
It is also a huge bestseller manga, it means a lot to many many people.
You may not like it, but it has already its place in mangas, animes, cinema and japanese history.
It is the opus magnum of one only man who devotes his life to create a fantastic world full of inspiring characters, motives and mithology.
Maybe the only problem is that is isn't finished yet, so we can't judge it as a complete work.
It is very difficult for westerns to truly appreciate a japanese art form as the japanesr do. The perception of time and the expectatives are very different.
You can live your whole life without One Piece, the music of J S Bach and the plays of William Shakespeare, but I can say to you, art can truly inspire your life if you just appreciate it, instead of judging it.