If you are the kind of person who dislikes a fighter like Demetrious Johnson because of their size you should understand that this is no different than you saying you dislike them because they are black or hispanic.
In both situation you have a person who cannot control a factor about themselves that you use to discriminate their character, except one is taken seriously and one isn't.
For example, if a girl was to go on a date with a guy and he was black and she said "I don't date black guys",
There would be an outrage.
But if she said "short guys" it would be acceptable.
Both are discriminating so don't pretend that you are progressive with race issues if this bothers you.
Making jokes about Demetrious Johnson based on his height should be treated just like race jokes, with appropriate punishments.