Continuous growth of data traffic in optical access networks is driving the increase of utilized data rates even up to 25 Gbit/s per wavelength. While due to utilization of signals with large electrical bandwidth, requirements on the bandwidth of electrical and electro-optical components increase significantly. Addressing that, the feasibility of application of the minimal transmitter and receiver bandwidth has received considerable attention. In this research, via numerical simulation in VPI Photonics Design Suite software we evaluate the impact of the shape of MZM frequency response based on amplitude frequency response (S21) from commercial MZM specification. Moreover, addressing the requirements on numerical simulation models that ensure simulation of MZM frequency response curve for prediction of the overall system performance, we have also evaluated several analytical frequency responses (corresponding to the responses of 1 and 4-pole, Bessel and Gaussian filters) with different 3-dB bandwidth values. Based on the obtained results, we propose the approach for the simulation of MZM frequency response that is the most relevant for our utilized PAM-4 modulated WDM-PON system with 25 Gbit/s bitrate per channel.