I took this picture to email to a friend of mine who is an expert breeder of Crested Geckos to find out if this lizard's under bite is indicative of a deeper problem or just possibly an anomaly about which I need not worry. I have fed it well using Rapashy Crested Gecko Diet, baby food such as peaches, pureed fruit and live insects (such as blaberous roaches). So, I am very doubtful that the condition is related to its diet. There is a chance that the condition was caused by this gecko fighting with one of its cage mates or due to some other type of injury but I kind of doubt it. I suspect it may be genetic in nature.
I am not all that concerned as the gecko seems to be in otherwise excellent health.
Other than the under bite, get a load of that eye ball. Their eyes always amaze me.
All the best,