I don't understand that line of thinking at all, but I can't speak for UFC fans I guess.
It shouldn't be any different than Emmitt Smith having a bad showing on Dancing with the Stars, or Drew Brees getting drubbed on celebrity jeopardy or something.
I mean, I know it's different based on the profile of the thing, but it's bizarre to me that UFC folks would turn on the guy for losing badly in another sport. If their appreciation of their fighters is based on the premise that they could beat the best athletes in other sports in their sports, that's an absurd fantasy (as it would be for any fans of any sport).