It is normal for some breeds but not others. It can indicate illness if it occurs in a chicken that normally has an upright crest. It can also indicate dehydration, the body being too long in a state of brooding, an attempt to heal an injury while not supported through diet sufficiently (in this case it represents a form of autophagy, wherein the body is consuming some cells to grow others back) and it can also represent hormonal issues/insufficiencies.
If the chook's always been like this then it's just genetic. Some breeds have this as a trait. I've seen this in white leghorns from families that included normal leghorn crests too. I don't think they were 100 purebred.