Just why do you think mats change the sound? Could it be because they dampen the record in differing ways? If so would they be dampening some sort of resonance? Where would you guess that is coming from?I knew I would get this type of response as I was typing what I typed. Substitute "deaden" for "dampen" and I can answer your question. Too heavy of a platter vis a vis the motor and bearing deadens the sound because of they-the motor and bearing-are over-loaded. Imagine a diesel tractor chugging up a hill with a heavy load. The same applies to adding too much weight and sound absorption with a cushioned mat and weights.
You can insist in calling it "dampening" if you wish. Speaking of vehicles, imagine a Porsche 911 loaded up with four 350 lb occupants. Besides the grotesqueness of the mental image, would you call the driving quality-besides being blatantly dangerous-"damped" or "dead".