We measured Raman spectra of several SiC polytypes (4H, 6H, 15R), which were isotopically enriched by substituting (12)C by the (13)C isotope. The lines of the different phonon modes of each polytype broaden for the isotopically enriched samples. Moreover, for a given isotope composition, the broadening varies for the different modes. We demonstrate experimentally that the broadening of the zone-center TO phonons due to atomic-mass disorder scales linearly with the phonon density-of-states as predicted in the framework of a mass perturbation theory. From the proportionality factor, we derive the absolute value of the phonon eigenvector of the carbon sublattice as \e(C)\ = 0.83 +/- 0.01, in good agreement with pertinent ab-initio calculations.