He was hated (the fictional character) cause he was a chicken – the same reason similar kids are hated everywhere. He got into karate to win respect, but he could have gotten the same thing by throwing a rock at a school window – no joke!
Now, I do think working out will increase the respect you get, but I suspect it isn’t enough. People are also into initiations like frats or gangs have. In other words, the person wanting to be part of the group has to prove themselves by doing something crazy.