Confusion in FFT of Resampled Data
As you know you have to use an ideal low-pass filter, and zero order hold is an approximation which needs more compensation. Ideal LPF has a flat response with sudden cut-off and zero stop band but your moving average window has a sinc frequency response. So the deep valleys belong to the zeros of sinc function and the remaining which has high attenuation passed from the stopband of your filter (which is not zero because it's not an ideal LPF).