I have no definite answer. It could be a mix of real and fictional elements. Maybe check with reviews or the filmmakers' statements to get a clearer picture.
I think 'dumb money based on a true story' might be about the stories of regular people who get involved in financial markets without much knowledge. They are the 'dumb money'. Since it's based on a true story, it could explore how these people were influenced by market trends, false information, or just their own naivete. It could show how they lost money or made unexpected gains in real - life financial situations.
In fact, 'Dumb Money' takes inspiration from actual incidents and characters, making it a true story to a considerable extent. However, some artistic liberties might have been taken for narrative purposes.