First, the picture is awesome.



My backstory for human!Pinkie: She never experienced anything like Rainbow Dash's Sonic Rainboom, so she's still in a deep depression. She only sees hard work in her future, without break and without joy. Her father was upset that she wasn't born male, so she tries to be that for him.



And then... Pinkie comes through the mirror.



She sees that her femininity is fine. She sees that the world does have joy. She sees that there is more to life than work.



Pinkie brought her out of depression.