Where did the song "Yume ni Katachi wa nai keredo" come from in One Piece?
Unfortunately, the song is not an official One Piece song.

It's a parody song of 歌に形はないけれど (Uta ni Katachi wa Nai keredo) by doriko (Vocaloid producer) with its lyric changed.

The original song: NicoNicoDouga (posted in 2008, the official video posted by doriko himself)

The parody song: NicoNicoDouga (first posted in 2010, the original/oldest video with Hatsune Miku as the vocal)