It outputs 2 identical mono channels of the mixed stereo signal. However, a stereo cartridge is set up to detect both lateral (side-to-side) and vertical (up-and-down) motion, because that's the way stereo records are encoded (at 45 degree angles on opposite walls of a V-shaped groove). A mono cartridge is deliberately set up to detect only the lateral component, because mono discs only have lateral encoding. Therefore the signal level of the output generated by a mono cartridge playing a stereo disc is theoretically not optimum, resulting in a lower signal-to-electronic-noise level for the amplified output. In practical terms, it sounds reasonably ok.
A stereo cartridge playing a mono disc also results in a weaker signal to noise ratio, for a different reason. The stereo cartridge will detect dust and vinyl imperfections in the vertical plane, rendering it as noise mixed with the lateral mono signal. A mono cartridge will ignore all vertical movement, which on a mono disc is not part of the music anyway.