I’ve never used the term fred, but I always thought that traditionally it was someone who didn’t have the “correct” look. Whether it was the clothes, or the bike, or the number of bags on a bike. It might even be someone wearing spandex to mountain bike. I would say, it would include not wearing a shirt.
But a fred was never a poser. A poser in any sport is someone who buys whatever is needed just to fit in, but not really wanting to put in the necessary effort, training, or practice to be at the level they are trying to promote.
It was infinitely worse to be a poser than a fred.
John