Is it true that smoothies made with pure fruit contain too much sugar, although natural, to actually be good for you?

Yes, sugar is bad: Drinking fruit smoothies, drinking fruit juice, and eating too much fruit, all lead to excessive sugar consumption. Natural sugar is just a bad for the body as "unnatural" sugar is, if the natural sugar has been separated from the plant fiber, or otherwise purified. The best thing to do is to eat very little sugar, and think of fruit as nature's candy or dessert.