In order to answer this question, we must first understand the dimensions of a regulation NBA backboard. Regulation backboards are six feet wide and three and a half feet tall. The top of the backboard is therefore three and a half feet off the ground.
Now that we know the dimensions of the backboard, let's consider whether it is possible to touch the top of it. To do so, one would need to be able to jump at least three and a half feet in the air. This is certainly possible for many people, but it is by no means easy. Professional basketball players can make such jumps look effortless, but they have spent years honing their craft. For most people, touching the top of an NBA backboard would be very difficult, if not impossible.
So, to answer the question posed: yes, it is possible to touch the top of an NBA backboard - but only for those with exceptional jumping ability.