I mean if he is the king of all dragons because the hidden world is basically the original home of all dragons doesn't that mean he his the king of all the Bewilderbeasts?
Yes. However, the Bewilderbeasts can fight each other over dominance. Not only this but the winner of a fight takes the others place a king. When we saw that white Bewilderbeast get killed by the black one, the black one became the king because he defeated the other. But when Toothless defeated the black Bewilderbeast, he became the king of all dragons. It’s like fighting for the crown. But taking that Toothless isn’t a Bewilderbeast type dragon, other types of dragons must be able to do the same thing. So at any point you if another dragon were to defeat Toothless in a battle for being king then Toothless would no longer be king.