What you are referring to is known as a "broken crest" and it suggests that at one point in time (probably before you had her) your donkey was overweight. Donkeys are REALLY easy keepers, and they are prone to gaining weight. Once the crest is broken (very thick) it will NOT reverse, even if your donk is now in very good condition. Don't worry, not a current problem for the animal, but it does point to a past over-conditioned state.
Just keep track of Molly's diet and keep her from getting over-conditioned again!
just have to clarify the terms here. What that is is a FAT ROLL. A BROKEN CREST is when the crest of a animal falls to the side. The FAT ROLL is where in hearty times donkeys store their extra fat for the lean times - this is how nature designed them to live in the desert - store up when the rains are there and use it up when the dry season is. Some donkeys get these VERY easy. My Hershey has one from before I got her. She also has the fat deposits on her rump which vary in size according to the season. If a fat roll falls off to the side (if they get VERY obese this happens) then it is refered to as the broken crest or a fallen fat roll.