That doesn't equal the best though, I could ask people who aren't into Pop if they know who Justin Beiber is and they will say yes. It means the mainstream are aware of them, it speaks nothing to the quality of their music or, in this case, them as characters.
People want Batman, a Robin, Joker, Harley. Those are who Batman fans will pick something up for, anything else is just icing. Batman, Robin, Joker, Harley, and Ivy/Scarecrow with the other as DLC and Mr. Freeze as a skin is perfect for casual Batman fans. Sure, you get people that want Croc and Riddler but they aren't gonna not buy the game because they're absent when these other Batman characters made the roster.
Really good, fine. Batman is no more serious than any other comic. Green Arrow's universe is in fact DARKER and more serious than Batman's, he just isn't a uber serious character. Superman has been ultra dark at times and he's seen as the beacon of shininess in the public's eye. Really good? Fine, but so do others. Why is Batman getting 8 characters when Superman, the #2 franchise ATM (and still their #1 franchise overall) getting 3 CHARACTERS? He doesn't even get his archnemesis when Boon himself said you can't have a DC game without Lex and Joker. Joker makes it, Lex doesn't. Batman gets his "doomsday" in Bane, Superman doesn't.
And trust, this isn't sour grapes as a Superman fan, it's sour grapes as a DC comics fan. When I think of the potential of a Mirror Master, Weather Wizard, Circe, Animal Man, Vixen, Larfleeze, Amazo, and so on being left out to fit in more Batman people is completely disheartening when I have to wait another 4 years just for the CHANCE of seeing someone I really want in to make it.