I don't mind the idea of Killer Frost being in the game, as I would like to see more characters in the DC Universe get some attention and raise awareness that DC has some pretty great characters out there.
But then she really didn't end up doing much, so ultimately the only awareness she raised is that she exists. In that regards, Captain Cold might have been a better choice since at least he could have gotten some interactions with Flash during Story mode.
On a related tangent, that's why I'm not a big fan of the idea of Scorpion being a DLC Character. If Scorpion was a hidden bonus character or something I really would have no problem with his inclusion at all (even if he was an unplayable boss character like the Reptile/Smoke/etc. fights in Mortal Kombat).
However, when it comes to the extra characters that you have to pay for who get a bunch of media hype and trailers...I would much rather that slot go to an existing DC character to help promote them (especially if it's a character who isn't from Gotham City so more people can be aware that DC has some pretty awesome characters beyond Batman) instead of a guest.