I've noticed it, especially in CRJ's. In those planes, the window seats seem harder than the aisles, FWIW.
Most of the time I fly Delta who across the board seems to have some of the most run down airplane interiors, even in FC. Their seats may have had padding a long time ago, but years of constant use have crushed the bottom cushion padding to death. I can almost live with that, though. What kills me though is their low-back style seats. Impossible for anyone over 5'6" to sleep in.
JB