if electric force is independent of distance r.would a neutral insulator still be attracted towards the comb.

There shouldn't be a force on the insulator. The reason for polarization is because the repulsive force on the like charges, when further away from the comb, is less than the attractive force on the unlike charges closer to the comb. Therefore the insulator becomes polarized. But if the Coulomb's law had no distance factor, the forces would be equal, and there woouldn't be any polarization, and so no attractive force.