Maxxx, this looks like a classic case of what we discussed previously in the other WIP thread of the rifle; geometry artifacts.  The reason for it here is just the amount of long, unsupported edges making up the curved portions of the cushion.  This is why we should strive to model with faces and edges of reasonably similar length and size.  Try adding one or two more edgeloops through those long, unsupported edges of the cushion, then apply a regular smooth modifier.

It also looks like you may have some unwelded verts in that center vertical edge loop.  The edge loop seems to  "split" up near the top.

______________________________________

My Store
My Free Models
My Video Tutorials
My CG Animations
Instagram: @luxxeon3d
Facebook: https://www.facebook.com/luxxeon