In most cases, no. American fiction is focused on the main narrative and doesn't commonly incorporate post-credit scenes. However, there might be some rare exceptions in experimental or unconventional works.
A post - credit scene scary story might involve a cursed object. In the main movie, the object seems harmless, but in the post - credit, it starts to glow and strange noises come from it. It then causes chaos in the empty theater where it was left. Also, there could be a post - credit scene where a character finds a diary that tells of a horrible past. As they read it, shadowy figures start to appear around them, and the words in the diary start to bleed, creating a truly terrifying moment.
It can be quite important. It might set up future plotlines or introduce new characters. For instance, it could be the start of a new arc for a character like Qi'ra.