MM doesn't mean Martian Manhunter. In comic book terms, Martian Manhunter is MMH, not MM. MM could possibly be Mr. Miracle. (or I'm taking this way too far and it is Martian Manhunter)
Let's be honest, did everyone know that Ares, Lobo and Killer Frost were actual comic book characters before this game? Any of those characters can be some random character that has a unique fighting style that fits this game. A lot of people complain that their character isn't in don't realize that they probably play really similar to others. Zatanna, for example, would play very similar to Raven and Martian Manhunter could play very similar to GL.
To put it short, people shouldn't complain about their character not being DLC because the guys at NRS have very good reasons for them not being in the game. Having character polls, however, is very unfair for the fans if they don't put the winner in.