Is it true if when you lay down your tummy goes flat you will have a better tummy tuck result? (photo)

Based on these photographs you appear to be an excellent candidate for a full tummy tuck with muscle reconstruction.  This will flatten the muscles, reconstruct your core muscular corset, revise the belly button, eliminate the stretch marks from the belly button to the pubic hairline, and remove the loose skin and fat of the anterior abdomen.

 Although definitive assessment/advice would require more information (consider posting frontal/side views standing as well),  I think that your statement does hold some truth to it. Essentially, patients who do not have excessive intra-abdominal (visceral) adipose tissue will likely have better aesthetic outcomes with tummy tuck surgery.   When you stand, the presence of abdominal wall muscle laxity allows for intra-abdominal contents to "bulge" causing the appearance that you do not like (protuberance of the abdominal wall).  Patients in your situation benefit from plication of the abdominal wall muscles and significant aesthetic improvement of their abdominal wall contours.

 Best to seek in-person consultation with well experienced board-certified plastic surgeons for accurate assessment and advice. I hope this, and the attached link helps. Best wishes.