When the Justice League set up a meeting with the new Ghost King, they had expected him to be a powerful, imposing figure. So when they entered and saw a gangly teenager wearing the crown, they assumed that he must have been chosen to be an easily manipulated puppet figurehead while things were actually being run by others.
And, to be fair, Danny is relying a lot on the advice of the other ghosts. He’s still very much young and inexperienced, especially compared to the older ghosts. But he’s definitely not their puppet, nor is he by any means weak.
Which means that the JL’s assumption can lead to some rather significant misinterpretations of things down the line…