I don't have a problem with it. If they want to dress like the opposite gender, well, good for them. I don't have the right to stop them, nor does anyone else.
I must admit though, seeing it can be quite confronting. I would never do it myself.
Applejack and Celestia are both pretty big options for me. Celestia's past with Nightmare Moon and dealing with the loss of her sister could be quite upsetting, but Applejacks *probably* dead parents would be a hard-hitter if that was touched on more too.