Whether the bosses are getting harder or easier depends on your perspective. I know that's kind of obvious, but Skyward Sword clearly divided Zelda fans in more than one area. I only have The Wind Waker, Twilight Princess, and Four Swords: Anniversary Edition to go on, but it seems bosses got pretty easy between Majora's Mask and Skyward Sword. They were essentially huge puzzles that could attack, and once you know the answer to a puzzle, it's just not as challenging.
Not all Skyward Sword's bosses were intimidating powerhouses, and yes, they could have been harder. I think what really separates them from previous bosses is that you generally have to be on top of your game to avoid getting slaughtered. The top of your game may not be very high off the ground, but you still can't slack, or you'll get whacked. (I just came up with that.) Ghirahim's battles aren't truly that difficult, but they require timing, precision, and a bit of sheer reflexes to boot. Look away from the screen to grab a pretzel or something, and expect to be face-down on the floor when you turn back.
I would say bosses were getting easier, but then Skyward Sword happened and they became more challenging again. They're still not quite where they should be, but unless Zelda Wii U returns to the whole glorified-puzzle-boss ideal, they're on the rise.