Twilight isn't really that important in that scene; Pinkie and Ditzy are more Twi simply asks the question and is an onlooker to the quarrel, so she doesn't belong to the foreground.

I drew her that way on the sketch, after reading the scene in the fanfic. I am aware of weird perspective, and at some point I thought to change it, but decided to stay with the original draft. Thanks for noticing this, so I could explain.