Carseat mats are considered very unsafe because they don't comply with car safety standards (even when supposedly crash tested):
That said, I have heard of techs who will use certain ones in certain circumstances. Their test for whether the one they have is safe is as follows:
1. First install the seat without the mat. Make sure there is less than 1" of movement in any direction at the belt path.
2. Take a piece of chalk and put a mark of the LATCH belt/seatbelt to see how far it needs to be pulled to be safely installed.
3. Uninstall the seat, put the mat on, put the seat back on and pull it. If you can get the chalk mark exactly where it was when the seat was installed without the mat then it's a tight enough install. If the chalk mark isn't where it was then the install is too loose and the seat cannot be ridden in.
Disclaimer that doing this is likely to leave worse indentations than not using a mat at all.