Sugar is cheapiest and easiest way for your body to provide fuel (glucose in blood) for your brain. Sucrose is made from binded fructose and glucose molecules. Of course you could consume fructose and even pure glucose, however, they are much more expensive (both financially and by energy needed to consume) and had lower coversion rate to the glucose in blood. Glucose from sugar is placed into blood during simple brakedown process by sucrase enzyme on specific cell membranes, but pure glucose have no such mechanism of consumption, and the best way to somehow put pure glucose in blood is not an eating it, but a direct injection into blood. Pure fructose also, unlike sugar, have to make a long and complex path of transformation into glucose in blood. That is why we have that literally dumb narrative "sugar is bad". Healthy person could eat sugar without any limitations and without any side effects. Any problems with shugar consumption is only a consequence of some body malfunction or illness. Broken body use sugar to do some weird things, instead of splitting it to glucose in blood and feeding the brain and other cells. It's not a shugar such person should avoid. It's just an organism breakage that need to be repaired.

If you want to make your population dumb, just tell them a tale about harmful sugar, and that's all.