Welcome!
I'm pretty sure (I've never seen it NOT in a Graco manual, anyway!), that a mat is allowed under that Snugride Click Connect (the seat you have). The Brica is exceptionally bulky, however, and actually does tend to make seat installs unacceptable. There's a blog here with a graph of ratings when you scroll down that's helpful...
http://carseatblog.com/21536/vehicle-seat-protectors-update-what-you-need-to-know/
Honestly? After years and years of super duper tight installs for my three kids, I find that dents in seats pop out, and food spills are easy enough to clean up. After barely ever using any seat mats, when I traded in my 11 year old van, the seats were the nicest part of the car
(The chocolate milkshake in the carpet, however? THAT never came up, no matter how much I cleaned, lol)