To answer your question I need to make a distinction in the type of TT done. A full TT elevates the skin from the lower abdomen up to the rib margins. This separates the abdominal skin from a part of its normal blood supply which comes up through the rectus muscles (the sit up muscles). The flap stays alive from blood vessels which run within the abdominal skin flap and usually does so very well. However, if in addition to full elevation of the abdominal skin flap, a surgeon runs a liposuction cannula into the abdominal skin flap to thin it, the blood supply to the part of the flap closest to the incision may lose some of its blood supply and die. This is why surgeons avoid liposuction of the upper mid part of the abdomen during a full TT.
There is a slightly different type of abdominoplasty where the skin is not completely separated from the underlying rectus muscles all the way up to the rib margin but this area is liposuctioned instead. Some blood supply from the rectus muscles is still feeding the flap so the liposuction can be done to thin the upper part of the flap safely and then the lower abdomen excess skin is trimmed off and the abdomen is flatter and tighter. This is called a Lipoabdominoplasty and gives excellent results in the proper patients. Contact a surgeon certified by the American Board of Plastic Surgery and get more details.