This paper presents the 2-D numerical solution of the flow and concentration field of an electrokinetic T-type micromixer, under heterogeneous zeta potentials modulated via sinusoidal functions and interfacial viscoelectric effects. Here, the viscoelectric effects appear to modify the fluid viscosity due to the high voltages within the electric double layer. The mathematical model is based on the Poisson-Boltzmann, mass conservation, momentum, and species concentration equations. In the steady-state analysis, two electrolytes with known ionic concentration and an imposed velocity profile are considered at the inlet of the micromixer. The results demonstrate that by using heterogeneous zeta potentials, at the mixer walls, generated flow recirculations along the mixer channel, promoting the rise in mixing efficiency; however, for high zeta potential values, this is counteracted by the viscoelectric effects. The present investigation shows how the viscoelectric condition deteriorates the mixing performance and how with the correct selection of modulated zeta potential parameters as the wave number, and the phase angle can improve it. Therefore, the performance of the mixer studied here should be considered for the design of microfluidic devices in the future.