This paper deals with the influence of the shape of the driving waveform on the frequency responses of MEMS resonators under nonlinear actuation. Our models show that, at large oscillation amplitudes, these responses are strongly dependent on the shape of the actuation waveform, so that the nonlinear frequency response is not the system signature but the system signature under a specific driving waveform. The case of a MEMS resonator electrostatically-driven with a sine-, pulsed-or square-wave voltage is specifically addressed. Our models and simulations, supported by experimental evidence, predict counterintuitive phenomena resulting from the distortion of the actuation waveform by the displacement-dependent electrostatic nonlinearity. Our study emphasizes that this issue should not be overlooked in order to perform quantitative MEMS characterization in the nonlinear regime.