The cartridge signal is small, in the sense of low power. That is fairly independent of the impedance level. For example, 5mV at 50Kohms (typical moving magnet) is 0.0005 microwatts, and 0.25mV at 125 ohms (typical moving coil) is also 0.0004 microwatts. The question about cable impedance is, what is the source of interference, or sonic compromise from which you need to protect the signal?
At high impedance, such as a MM cartridge or the output from a SUT, the biggest interference risk is electrical fields, which is addressed with good shielding. The biggest signal damage risk is cable capacitance, which will corrupt and/or dull the treble. So you want a short, low-capacitance, well-shielded cable, and the cartridge or transformer secondary would benefit from electrical shielding as well. Cartridges with metal bodies and transformers with internal and external shields are designed with this in mind.
At low impedances such as an MC output, the interference risks are ground currents and magnetic pickup. These are best addressed with twisted pair, or even star-quad, wire and balanced lines. Decades of studio and PA experience with microphones shows that tiny signal can travel long distances this way. In the absence of balanced lines, great care must be taken to avoid ground currents by allowing the cable to be grounded at only one end, and making sure that there is little capacitive coupling of power-line voltage to the shield at the other end.
Of course these are the extremes, and it's always useful to minimize ground loops even at high impedance, and to use good electrostatic shielding even at low impedances.
A final note - the SUT must be either magnetically shielded, or kept away from power transformers, or both.
