I love the music in Yoshi's Island. It was much better, in my opinion, than what was prevalent in Super Mario World, but those two games were different beasts.
However, Yoshi's New Island has gone above and beyond the call by having some of the worst music I've heard in a Mario-based game. I sure hope this isn't a sign of things to come from this title. Take note at 0:35 and at 4:58. These are especially awful. The main track, from Yoshi's Island, is redone at the beginning of the game and even it sounds off.
Yoshi, you deserve more!