BBL created an ugly, squared, butt crease. Can it be fixed, if so, how? (Photos)

I think this can happen on occasion from the weight of the augmented buttock.  Usually though, a rounded crease stays round and a flat one either stays flat or rounds out depending how the fat was placed.  Transforming it from flat to round is not easy although it can be attempted.  You'd need an in-person consult to assess the tissue quality and chances of seeing the result you seek.  

Ideal candidates are at a stable weight,  in good health , and have excess fat in some parts of the body and too little in other regions.  Gaining weight before the procedure is to no advantage and when you lose weight again the fat cells will shrink just like the donor site fat cells.  Typically, the transferred fat results in a significant increase in volume of the body site being treated.

Part of the BBL procedure is to shape the buttocks and there are limitations to this. It's hard to know what happened without a before picture but your PS should be able to perform further fat grafting to improve this area.

First, you should be using a boopy pillow to sit, rest and sleep, it will be very helpful to prevent your shape to change, besides that, use your garment every day, it will help too.