In a game where characterization is dictated primarily by animation, it's hard to make the more "realistic" characters as interesting as the cartoonier ones. Snake and Cloud are great, complex characters, but you aren't going to get in their head in a fighting game with a couple of lines of dialogue. That said, because of my attachment to those characters, I still really enjoy playing as them.
On the other hand, I gravitated instantly to Duck Hunt back in 4 due to his cartoony nature, compared to all the other humanoid newcomers. Most of my requests tend to be on the cartoony side of things as well (or less generically humanoid in general), and my favorite Nintendo series are Mario, Pokemon, Zelda, Donkey Kong, and Kirby... making Zelda the odd one out.
I think the advent of Ridley has introduced the exciting possibility of more "realistic" non-humanoid fighters. His design in Smash definitely stands out in comparison to just about every other fighter. The contrast of a more realistic looking purple dragon doing goofy cartoony stuff in Smash is amazing.