Yes - fleece is man-made polyester - and is desirable for your pigs, as their urine will no sit on the surface, or keep the fleece wet, but will 'wick' through to more absorbant layers beneath. I think one of the mods - was researching into what fleece performed best, I know the fleece I bought off the bale at a material shop works better than the throw I bought in a 'home' shop.
Wash the fleece first eitherway, but, yes, it is OK!