The goddess harp was a great idea on paper. The harp is a very beautiful instrument (both in appearance and sound), playing an instrument is a tradition in Zelda games, particularly the 3D ones, and players generally love doing this. But it came up short. For one, the player is not actually playing the instrument, they're just swinging back and forth. This creates a tedious sensation, especially if you're a veteran Zelda player and have had experience with OoT's ocarina, WW's wind waker, and ST's flute. Even TP's howling made the player feel like they were responsible for the noise that was coming out. The only time you were actually playing the harp was during the late night concert in the Lumpy Pumpkin. The next issue is the music itself. Yes it sounded nice, but a general complaint about SS is that it's music isn't very memorable. This complaint is included in the music you must play. A game does not have to have memorable music, but Zelda is known for this. Past Zelda games have gotten songs stuck in players' heads (in a good way) always making us appreciate it moreso than other series appreciate their music. And it doesn't help that you can only play actual songs in specific locations and that you only play them once.
The harp isn't the worst thing in SS and if you've never played a Zelda game you might not see any faults in it, but there are definitely ways it could've been better. Again, a great idea, just lack luster performance.