Well, I'd say who's most
"unfairly" disliked is a matter of opinion. I've seen hate mostly on the Mii Fighters, Dark Pit, and more recently, Little Mac and Diddy Kong. A little bit on Wii Fit Trainer, but mostly because that came from guys I know who don't like to play as her/him.
![Mii Male :4mii: :4mii:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
I'm a big fan of the Mii Fighters, so it's a bit depressing to see them get hate. You can make a case for banning them in competitive play, possibly, sure, but I don't think that should justify the hate they might get. Personally, I think it's fun to make your own characters and play around with them. Maybe that's where the Miis lose appeal with some people, I'm guessing.
![Dark Pit :4darkpit: :4darkpit:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
Ever since Dark Pit got revealed, I've not seen the hate for him stop. Many say it would have made sense to bring Palutena from Kid Icarus and let that be it, and I get it, since Dark Pit is a more obscure character, but I personally actually kind of like his character, and I use him every now and then. More than Pit and Palutena combined, at least. My personal vote would probably be for this guy.
![Little Mac :4littlemac: :4littlemac:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
I don't see Little Mac get hated on as much anymore, with the advent of the "Hoo-hah"s dominating the irritation in competitive Smash players now. I haven't played against him very often, so I can't claim whether others who have trouble with him are bad or not, but after checking the Little Mac boards, most people say it's not as easy to effectively use him as people think. With a character that's this bipolar, I can see why.
![Diddy Kong :4diddy: :4diddy:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
I've never played a competitive Diddy, so I'm not on the bandwagon. If everyone gets wrecked by him, it'd probably make sense to see people get mad, but I've seen lots of discussion surface saying that he's really not that bad, so, Diddy doesn't need the hate, I think.