I never really have expectations for bosses that aren't endgame ones. Because of that, I didn't find Morpha to be a disappointment. If anything, I was too busy staring at the screen in surprise at what it is. It was also rather unique for a boss, all the others are usually big monsters or something fierce, this thing was so "out there" that you didn't really know what to expect. Because of that, he caught me off guard at first. Although, I can understand if you went into the boss room expecting some big sea-monster or something but instead, you get... That.
Of course, now it's more of an annoying boss to me, since I seem to have horrid luck with the longshot actually hitting it. Which winds up causing me to lose way more hearts than I intended. Although, I guess it's still unique since, in my opinion, it's the only annoying boss in the game.