I think each character can stand in their own right, whether male or female. When they have a wide choice though, I think it's legitimate to ask why they chose mostly male characters.
The hero cast has been partly dictated by the origin movies that have come before, with only Hawkeye being new, so I agree, Mockingbird would be a good choice. In fact, they could have introduced her as the SHIELD liason in this one ready to tool her up in the next one. Either that or introduce Sharon Carter as the liason. We already have a male agent in Nick Fury, so making the superfluous character a man as well is where they went wrong in terms of balancing the cast in my view.