The determination of viscoelastic property variation in short fiber-reinforced composites, as a function of strain, temperature, and frequency, constitutes a useful tool when the existence or the strength of matrix-fiber interfaces are to be examined. In this work, these properties are studied in thermoplastic elastomer matrix materials (SBS) filled with commercial carbon fiber (PAN), oxidated, and subsequently treated with diazide, which generates SO2N3 groups on the fiber surface, theoretically capable of chemical reaction with the polymeric chain. For the composite containing diazide-treated oxidated fiber, certain phenomena have been proven to occur, such as lesser storing modulus losses in case of increasing strain, greater equivalent interfacial thickness values, and higher mechanical energy loss values than those of composite containing oxidated fiber. An increase in glass transition temperature and apparent activation energy of the relaxation process, with respect to composite-containing commercial fiber, are also observed, which, altogether, allow for the statement that new matrix-fiber bonds are generated through the sulfonyl azide group, conferring greater strength to the interface, although it is less stiff than the one formed with oxidated fiber. Finally, the experimental results are in agreement with those obtained from Huet’s theological model.