I agree entirely--the mat isn't needed. But think about it...you're putting glue which only becomes sticky when it melts due to heat....on top of...a heating element. Trust me, I work with hot glue on a regular basis, and it doesn't take a whole awful lot to make the stuff get gooey and meltey, and that would be REALLY bad if the heat spiked somehow, melted the glue and the snake got under the mat somehow into the melted glue.
Good luck!
Edit: I'm not sure if the heat from the element would be enough to melt it, but in the case of a malfunction that would be my worry. The stuff leaves blisters on me, and I'm warmblooded. I don't wanna think about how it could hurt the snake.