@IncroyableSauce Sukuna was basically Curse Jesus, basically the indisputable pinnacle of technical knowledge and skill in the field. Whether or not he is the most powerful could be argued, but NOBODY knew more about curses than him. Not even Gojo.
People take the moniker “King of Curses” to mean the “King of Evil” which while yes it can be interpreted that way its more an acknowledgment that he is the ruler of this power system and his understanding of it is second to none.
This isnt something he calls himself to be haughty either, its something ascribed to him.
Similar to how he is the Honoured One as ascribed to by the narrator. Whereas Gojo ascribes it to himself, which is a neat bit of character work imo that tells you a lot about how they view and interact with the power system.