Three weeks from now, when the grass starts to grow, what are you going to do? You really can't leave that bag as in the picture (what are you going to do, weedwhack around it?), and if you just throw some mulch on top of the grass it will grow through (grass is determined stuff). But the solution is pretty simple, too: before you put your bag of dirt down, put down a layer of 6-8-10 pages of newsprint all over the grass where your bed will be. Then put your bag of dirt down, then mulch all the gaps. By the fall, you'll have killed your grass and made the dirt below much softer. I recommend "Weedfree Gardening" by Lee Reich, for more on this sort of approach.