Ashley's excuse here is that successful snark from a character who has to literally look up while doing it can be more fun and impressive than a character who is towering over everyone in the scene.
That said, Susan has no difficulty being being both tall and impressively snarky, so it's possible Ashley really just wanted a shorter character with some curves.
I suppose it could be both reasons.
As for the shorter hair, I felt the ponytail was redundant with not-Elliot, and found myself interested in seeing Susan with shorter hair. When I threaten to give my characters with long hair short haircuts in actual canon, there are threats of torches and pitchforks, but all bets are off with Ashley's Original Characters, darn it!