A lot of people seem to have trouble with the harp! I never had a problem with it. You just hold the wiimote on its side (so the edge is facing up) and sweep it back and forth like a guitar pick. Maybe it’s because I’ve had a lot of practice with Guitar Hero/Rock Band. :)
And I actually liked the harp quite a bit. Not as fun as the ocarina, since you can’t actually “play” it. But I thought it was way better than the wind waker or that stupid wolf howling thing from Twilight Princess. I also am amazed that they managed to program accompanying arpeggios to all of the songs in the game for when you decide to whip out your harp and strum along.
I only tried the lumpy pumpkin sidequest once, and I’m not sure if I actually succeeded, but if I didn’t it was because the beat kept shifting.