Whilst reducing the aperture of a lens usually reduces aberrations and hence improves the flatness of the MTF, there is an optimum aperture for any lens and image sensor size beyond which smaller apertures reduce resolution because of diffraction, which spreads light across the image sensor.
While reducing the aperture of a lens generally minimizes aberrations and improves the sharpness of the image (MTF), there's a specific aperture that works best for each lens and sensor size. Beyond this point, smaller apertures degrade resolution due to diffraction, which scatters light across the sensor.