No, it is not. There are some economical mono cartridges such as Grado and Denon.
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Caspermao, it may depend on the mono records you play.
If the LPs you own are newer mono reissues, or any monos pressed since about 1970, a mono switch should be fine. Or you could insert Y connectors like Art Dudley suggested in Stereophile. But if you own original pre-1965 mono LPs, you will most likely benefit from a true mono cartridge.
Unfortunately it gets a little confusing but it relates to the cutter head used for the master. Original mono cutter heads cut wider grooves and had lateral motion only. True mono cartridges read only lateral motion, and they may come with different tip sizes. For quietest playback and best tracing, your cartridge should match the cutter. So, use a true mono cartridge with at least a .7 mil stylus for older mono LPs.
Sometime after 1965 (this may vary from one label to another) all cutter heads were a stereo design, even for a mono signal. This continues with current mono reissues. Therefore the cartridge need not be limited to lateral motion only. So a stereo cartridge can work. And many have found that modern, narrow profile stylus tips work very well on reissue mono LPs.