The game is great, but the music is pretty much shit. Atleast compared to the masterpiece of past Mario games' Koji Kondo compositions.
On second thought, the music is shit even taking out the comparison. Nintendo's DS games have been great, but musically, they have been lacking IMO. MK:DS pretty much had some of the worst sound/music design for the series and NSMB's music is at times grating. I turn the music off and play MP3s instead.