As for the thread- I can't think of anyone I really stopped liking, really only the opposite:
When I first started watching, I thought the Diaz bros were jerks and didn't like them. Then I heard their interviews and backgrounds and reasonings and now they totally make sense and I like them.
I used to think Diego Sanchez was so annoying but his fights with Kampmann and Ellenberger made me a big fan. The dude has so much heart and even though he probably should've lost the decision against Martin both fights made me love him
And Bisping has grown on me recently as well.
I guess if I had to pick one guy, maybe Mayhem- but it's more concern than dislike. He really seems like he lost his mind.