Can coughing damage or pop the sutures used for muscle repair?

At this point in your recovery, I would not worry about the strength of the sutures or the healing of the muscle repair.  Although a cough could potentially compromise the muscle plication, cough is sometimes unavoidable.  If there are no bulges you are likely fine.  

Generally speaking, it is difficult to "tear" a well performed abdominal wall plication.  If this does occur, it is usually associated with significant discomfort, swelling, and/or bruising. 
  Best wishes.