Who does Dorian represent or Sera, Josie and bull. They are all available to gay, lesbian and bi people. If a trans women is in the game and we are told that there is a wonderful representation of us but we're going to force you to play a gender that caused you extreme pain through your life, to be able to romance her I would be extremely disappointed and upset with BioWare.
No I don't think there is an issue with Cass being straight, I was disappointed yes but that's on me.
They represent people of their orientation. When people say "representation" in this context what they mean is the inclusion of these characters into the fabric of the world. So by having Dorian and Sera in the game, for example, we have (somewhat to totally) openly gay characters. They represented not through their romance content but through their being part of the game world.
Mae would represent straight transgendered women, in the sense that she would illustrate that such characters exist.
I'm also confused about why you feel that her availability would be representative. Let's suppose she's bi. You'd romance her as a biological woman, since DA doesn't really support gender identity variation for the PC yet. I'm just having a hard time following where Mae's sexuality intersects with her gender identity in a way that's important to you.