Utilizing non-local thermodynamic equilibrium time-dependent radiative-transfer calculations, we investigate the impact of mixing and non-thermal processes associated with radioactive decay on Type IIb/Ib/Ic supernova (SN IIb/Ib/Ic) light curves and spectra. Starting with short-period binary models of ≲5 M⊙ helium-rich stars, originally 18 and 25 M⊙ on the main sequence, we produce 1.2 B ejecta which we artificially mix to alter the chemical stratification. While the total 56Ni mass influences the light-curve peak, the spatial distribution of 56Ni, controlled by mixing processes, impacts both the multiband light curves and spectra. With enhanced mixing, our synthetic light curves start their post-breakout re-brightening phase earlier, follow a more gradual rise to peak, appear redder and fade faster after peak due to enhanced γ-ray escape. Non-thermal electrons, crucial for the production of He i lines, deposit a large fraction of their energy as heat, and this fraction approaches 100 per cent under fully ionized conditions. Because energy deposition is generally local well after the light-curve peak, the broad He i line characteristics of maximum-light SN IIb/Ib spectra require mixing that places 56Ni and helium nuclei to within a γ-ray mean free path. This requirement indicates that SNe IIb and Ib most likely arise from the explosion of stripped-envelope massive stars (main-sequence masses ≲25 M⊙) that have evolved through mass transfer in a binary system, rather than from more massive single Wolf–Rayet stars. In contrast, the lack of He i lines in SNe Ic may result from a variety of causes: a genuine helium deficiency; strongly asymmetric mixing; weak mixing; or a more massive, perhaps single, progenitor characterized by a larger oxygen-rich core. Helium deficiency is not a prerequisite for SNe Ic. Our models, subject to different mixing magnitudes, can produce a variety of SN types, including IIb, IIc, Ib and Ic. As it is poorly constrained by explosion models, mixing challenges our ability to infer the progenitor and explosion properties of SNe IIb/Ib/Ic.