i might need a check of my gamma correction but the monsters look to read not that easily over your background.
They don't, not at the first glimpse, and not static like in this screenshot.
I do, though, think that games have a tendency to make the sprites stand out a bit too much - since they'll move anyway, the eye will have no problems recognizing them in the first hand, without need for saturated high-contrast sprites.
I mean, look at the game Mad World, has equal contrast everywhere, but when it moves you can make out everything.
But in an ideal world, yeah, maybe a bit more contrast on the enemies (: