Nope.
It is quite relevant.
He was an active member of a citizen on patrol group for his neighborhood. He is not law enforcement, but he is out in the public's view to ensure the public is kept as safe as possible.
He is a public figure.
I was right, completely irrelevant.
Docent at a museum - you advertise yourself as improving society by contributing to the arts - public figure.
Teacher at a private school - you are in public view as responsible for children's education - public figure.
Own a restaurant - you are in public view as a supplier of food, something essential to life - public figure.
Make a citizen's arrest on a shop lifter - protecting public safety - public figure.
Teenager advertises availability as babysitter - taking care of kids - public figure.
All of the above are better arguments than the one you made, but still wrong. Zimmerman simply did not make himself a public figure by joining the
neighborhood watch. The media made him a public figure by 1) making a big deal about the "stand your ground" law and 2) portraying the case as a white-racist-kills-black-guy (and he isn't even white). I'm not claiming NBC should be liable, just that they should not benefit from the actual malice standard when the alleged defamation is a contributory cause of his status as a public figure.