turntable mat- helpful, harmful, or doesn't matter

Or is that to drown out the questions from Federal Regulators?

Come on now Marty let's quit picking on poor litle ole' Martha...

John the VTA is analog-speak for Vertical Tracking Angle. It's the angle of attack between stylus & record groove surface. By addng a mat, effectively raising platter height, the VTA changes which changes the sound. Many better tone arms have adjustable VTA for optimizing sonics; presumably yours may have this feature with your VPI/JMW arm (unless perhaps you have a Rega arm, which isn't adjustable in factory form). Lots of information is available on properly setting VTA; these posts can also be found here by using the forum archive search engine.
So what I'm saying is that all mats have different sonic signatures by themselves, but by adding mat height which changes the VTA, there is an audible effect there also. In order to hear the effect of the added mat alone, you'd have to measure your present VTA (& assuming it's already been optimized by whomever setup your arm) then adjust the arm height for that same VTA. Make note of where you started out (count the turns on the dial etc) so you can always get it back to where you started.
For the uninitiated, here's a link to George Merrill's outstanding & inexpensive turntable setup booklet:

How To Setup A Turntable
There you can also learn how to optimally adjust your VTA.

Don't worry about any smooth surface scratching your records, all the information is down in the grooves. If you're caught in a sandstorm, wait 'til it passes and wipe the platter before you place another record, however.

If your tonearm is flat, you can get an idea of the angle by noticing the difference in height between the pivot end, and the stylus end when the stylus is in place on a record. If you stack more stuff under the stylus end without moving the pivot end, you'll be changing the VTA. You might cause that exaggerated sssssssss sound on vocals, called sibilance, among other sonic flaws.

Carbon fiber turntable mat, hmmmmm, that's cool, but I don't know whether it will improve the sound over your stock platter, which is pretty well damped as it is. If you had an all-acrylic platter or a platter with ringing problems, like some aluminum platters, then I think the carbon fiber mat is a great idea. You'll have to let us know what it does for the VPI. And no, the "hard" VPI platter won't scratch your records unless it is contaminated with dirt or other hard particles of some kind. It's never happened to me, anyway. Just wipe it off once in a while if it gets dusty.

But I'm curious about this because such a mat could have other potential applications. How much was it and who makes it?

I have a carbon fiber mat that is paper thin. Think that I got it at Radio Shack in the 70s.