Black Manta is entertaining BECAUSE he's a one-dimensional psycho. He's a horror movie monster that goes through absurd hoops solely so that he can fuck with Aquaman, and he's consistent with it.
Joker has become the embodiment of "tryhard". I don't hate all versions of the character, but the modern comic/DCEU version is godawful. For what reason would he cut off his own face? How would that even work? How does that fuck with Batman? It doesn't, and that's the problem. He's edgy for the sake of being edgy and nothing more. Black Manta isn't even edgy, he's goofy and a borderline Silver-Age villain. He's ironically more like the Joker than the actual Joker.