Gotta imagine Harley being in Justice group shots as like, any time the Entire Marvel Universe is all in one big giant two-page spread and Deadpool is doing something silly in the background. Or frankly, more like Howard the Duck being in some little background corner. Although there's a part of me that can apply some logic to it for the same reason why I always believed she was a perfect character for the Suicide Squad.
Harley is a premiere criminal psychologist, who went crazy + was made crazy/manipulated by one of the preeminent O.G. super-villains, and has worked with a LOT of them. She's an expert. You could put up with a lot of her weirdness to get access to that level of insider knowledge and expertise. And in a room full of those people there's probably not squat she could do to cause any real mischief. (Let's all ignore Heroes In Crisis having ever happened).
Oh what else.
Mongul has been a bit of a petulant baby since Johns started using him. Tomasi, Bunn, and even recently Morrison have furthered that train. But its okay. I like him being all about ego and warfare. It does distinguish him from Darkseid for sure. Not that they should be confused for one another ever. Darkseid is Cosmic Satan and Mongul is just a Space Warlord. But they're both big, stony, with evil eyes and Death Star-type planetoids. So certainly one can see the comparison. But I love Mongul. He's got the "tries to be clever but fails" angle. The Black Mercy. The War World. Gladiatorial combat arenas. The family drama! His dad? His son? His sister? His just ... embarrassing ... rivalry with Sinestro. The fact that he defeated the Trinity in their own house, only to be bested by ... Jason Todd. He's just that annoying Space Tyrant Wannabe that won't stay down. Persistent, obnoxious, dangerous, doomed to fail over and over again and never win. But he's so tenacious. God, he's almost like a Marvel villain.