Is it advantageous to workout my butt prior to BBL?

Building muscle will not detract from your results. If you lose a large amount of weight, you might have less donor fat to transfer, but building muscle in itself will not negatively impact your results.

It's less about your muscle mass and more about how much available fat you will have. If you work out a lot you will probably have less fat to suction which will ultimately dictate the size of your buttockssurgical technique also varies greatly in terms of how much fat is harvested. Finally, the technique of fat transplantation varies tremendously and will dictate how much fat survives. Best of luck with your BBL.