I think Nighthawk hit on a good point about the fact that these are portraits. Portraits were expensive to have made and I'm sure that it would have been important to be dressed to the nines and as perfectly as possible. That implies to me that the way the kilts look in portraits are the ideal for the time not the norm. It would have been worth taking the time and effort to get the pleats just so if you knew how you looked was going to be preserved for posterity. For day to day dress, that may not have been the case. I know I certainly dress much better than normal when I'm having a formal picture taken.