Yeah, but both Oda and Mashima idolized him enough to base their own styles after his, which both eventually evolved but still have their similarities due to having the same 'base'. Frankly, I really don't think Fairy Tail and One Piece are all that similar at all. Luffy was hardly the first 'laid back, simple-minded protagonist with an iron stomach' and even Luffy and Natsu have things that set them apart. I laugh at Nami/Lucy comparisons because the only things they have in common are big eyes and tits.
The problem is, Fairy Tail is so generic that it can really be compared to quite a few series. Still, I like it enough.