Not a thumbs up, it was a guideline back in the day that if the mushroom cloud was smaller than the thumb of your outstretched arm, you were not in immediate danger. Or something along those lines. Pedantic needs fulfilled.
It's both. In episode 1 someone asks the cowboy actor to "do the thumbs up". Later, the actor explains the mushroom safety part to his daughter. In episode 3 there is another flashback where the cowboy actor does a photoshoot for Vault-Tec. He asks the photographer "what if I try a thumbs up?"
