My ‘18 TRDP hasn’t gotten much over 15-16 mpg since I’ve had it. I’ve heard that after the car is “broken in” it should improve. Could be a myth so I’m not holding my breath.
If you are only going to have the car seat in for a short time you should be fine with no protection. I have 9 month old, however, and will have a car seat for the foreseeable future. I purchased a Chicco car seat cover on clearance for $10.