Using LTspice to simulate common-mode signal interference in a differential amplifier?
You can measure CMRR in spice the same way that it is measured in the real world. Below is how analog devices measures there amplifiers, the same setup can be done in spice. It will only match the real world values if the model matches the real world closely. Where the amplifier in the schematic below says DUT, this is where one would plug in their amplifier model. Once CMRR is simulated correctly, the CMRR of the device can be modeled.

The test circuit is ideally suited to measuring CMRR (Figure 6). The
common-mode voltage is not applied to the DUT input terminals, where
low-level effects would be likely to disrupt the measurement, but the
power-supply voltages are altered (in the same—i.e., common—direction,
relative to the input), while the remainder of the circuit is left
undisturbed.
In the circuit of Figure 6, the offset is measured at TP1 with
supplies of ±V (in the example, +2.5 V and –2.5 V) and again with both
supplies moved up by +1 V to +3.5 V and –1.5 V). The change of offset
corresponds to a change of common mode of 1 V, so the dc CMRR is the
ratio of the offset change and 1 V.


Source: https://www.analog.com/en/analog-dialogue/articles/simple-op-amp-measurements.html