Mat doesn’t have to be the same hardness as vinyl.
Many people don't understand that the mat can have a big effect on the sound of the turntable.
The function of the mat is two-fold: first to damp the LP, and second to damp the platter. Both are important.
To damp the LP, this means that vibration from the LP must be absorbed and converted to heat. To do this without reflecting any of the vibration back to the LP, it must be the same durometer reading as the LP (the same hardness).
To this end you might use a stack of LPs but they aren't the right shape. Most LPs have a lip on the edge and a raised label. That is where a proper platter design pays off.
If the LP is properly damped you won't hear much sound coming from the LP surface (with the volume turned down) although this is not a 100% best way to know. To do that, a 20Hz-20Khz sweep tone does the job- it should play back nice and flat but distortion should be lower as well, since the LP talking back to the stylus will be creating distortion.