In the TV show, they would be unstoppable.

I honestly believe that nobody can seriously team-up with the Joker. He has his own agenda and would be a threat to everyone. That's why you see The Joker double-cross Black Mask in "Under The Red Hood".  
 
But, yeah, I think it would be a great episode. :)

That would be pretty goood, Black Mask could pose as a threat.

@Nigel90: I'd be worried about Robin. I mean, did you choose those two for their most notable similarity?

Its amazing that Joker was even part of The Injustice League, seeing as how he doesnt really "Play well with others." as the saying goes

Black Mask is a cold-blooded, sane murderer/torturer who makes people eat other peoples legs, arms, noses, etc...how can he be fit into a children's progam?

@duderkyle: By that logic, the Joker is a mass-murdering, sadistic psychopath who CUT HIS OWN FACE OFF. All villains can be toned down (or up) for whatever medium they're being put in.

@Nigel90 said:

In the TV show, they would be unstoppable.

Yeah, 'cept for all the superpowered characters.

@VenomX10: he tried to kill everyone at the end of the episode so .. he was in his note.

It'd be pretty cool.

it would give the kids nightmares

Black Mask would be awesome. <.<

While I like the idea (even though Joker doesn't work well with others), they'd get taken out easily.