I wouldn't take a mat. I didn't encounter many albergues that would let people sleep on the floor..... apparently, in the last year, the police have cracked down a lot on this because of fire hazards and if they disobey they can be shut down. I think a year ago it was different, but now you probably won't be able to sleep on a floor so the mat is useless. And, if you do find a floor to sleep on, likelihood is that someone around you will have a mat that you can borrow. Just about everyone I saw on the camino with a mat got rid of it or mailed it home at some point.
I went in April/May of this year and only brought a silk liner. I never slept on a floor, and only 2 nights out of 36 did the albergues not have a blanket (Roncesvalles and Ribidaiso) , so I was really happy to not have a sleeping bag, as it wasn't necessary at all. If you are cold at night, just wear all of your clothes and you will be ok, it shouldn't be very cold yet that time of year, although it does get much colder at night than during the day. Most people bring sleeping bags, but you really don't need them at all, just a liner.
buen camino!