Is Quic Disc Simply Carbs mostly simple sugars?

Yes, the formulation is intended to provide high glycemic index simple carbohydrates that your body doesn't need to do much processing to use.  The vast majority of the carbohydrates come from sucrose and dextrose with a relatively smaller percent coming from the maltodextrin.  There is a huge body of research that shows when exercising the body benefits most when the type of carbohydrates an athlete eats are simple.   There is also research that shows that combining multiple simple sugars together increases the positive effect on the athlete as each type of carbohydrate is processed by the body in a slightly different way, thereby allowing these multiple systems to work together on the different carbohydrate sources.   There is yet other research that shows a carbohydrate mouth rinse positively affects exercise.  Quic Disc Simply Carbs checks off all of the boxes.  Links to a small sample of the available research is found below.