You probably wouldn't be. I love sleeping quilt style - I do it in my hammock, or on the ground.
Yes, a mummy sleeping bag may stay warmer on you when done up. but I don't find it that much
different. Especially if you sleep in clothing, which in the cold, well, I always do.

Case in point, Saturday night I was on the ground (above treeline - boo hoo, no hammock for me) in Kananaskis - Not sure what the overnight temperature was but my playpus bottle had a good pile of ice in it in the morning and the person I was with who didn't blow out their tube had a nice solidly frozen drinking tube. (My guess is it hit about -3 to -4 C) I was on a blue foam pad, with a -3C rated mummy bag, that I had only zipped up the footbox, and had over me quilt style. I was wearing socks, long johns, a merino wool shirt, fleece, and a toque - I was quite comfortable. I roll a lot side to side when on the ground, and I didn't have any trouble.