It usually shows up in My Little Ponies who have dirt and build-up inside their bodies. I don't think it can be fixed, but you can prevent it by scrubbing out the inside of your ponies with soap and an old toothbrush. Just pop the head off with and X-Acto knife. The blotches of discoloration are caused by poor manufacturing. When a batch of vinyl is made, there is always excess leftover.
Some companies remix the leftover vinyl into new batches.
This most often appears in Lady Lovely Locks dolls. There is no way to prevent it or fix it. All you can do is hope that your new MIB doll is not a victim of rehashed vinyl. As stickers and decals age , they glue dries up and they peel. Take a piece of double sided tape and reattach the sticker to the surface.