I always say I can't judge people when I don't know them, but still you can have a (un)friendly feeling towards them. But I will never say "mario is not nice" as I don't know, it's just that he's one of the players I usually cheer against.
Sometimes seeing a player at a tournament helps to like them. For example I didn't really like Djokovic or Baghdatis, but when I saw them at a tournament I changed my opinion about them and now I regularly cheer for them in their matches. But that didn't help for Mario
I don't mind if people like Chiudi or not, I don't mind them to tell me they dislike him, or cheer against him when I'm around. But they should say stupid things about him, coz then I can get mad. Also coz I know Chiudi is really friendly. But so far I never met someone who doesn't like Chiudi