A pickup transformer is used to step up voltage about 10x. Of course, this also provides an impedance transformation. Source impedance of a MC cartridge is much lower than a MM cartridge, so it works out. Most MM cartridges are loaded with 47KΩ, but the best load for a MC cartridge is usually between 100Ω and 1KΩ.

Since a transformer giving a 10-fold voltage increase provides a 100-fold impedance increase, the typical 47KΩ load is transformed to a 470Ω load on the cartridge. You can decrease load impedance by adding a shunt resistor, or increase impedance by decreasing the turns ratio of the transformer.

Moving coil cartridges are usually pretty sensitive to load impedance, so it is worthwhile to match load to whatever the manufacturer specifies using a shunt resistor. You might even try a few values above and below the manufacturer recommended load to see what sounds best.