The first half features a fight against giant insects called "Sutchies". Many viewers on Reddit found this sequence to be "obvious padding" that dragged on too long before the real plot started.
The second half, which includes the character moments with Nekomamushi and the music, is far better received, being described as pleasantly "on-model and well-directed". Despite its animation shortcomings, the episode's narrative strengths and intriguing world-building manage to shine through.
One Piece Episode 765: A Musical Journey to the Whale Forest and the Shocking Truth About Sanji