Does a short crested cockatiel mean it is unhealthy?

If the rest of the bird's feathers are fine, then the cockatiel is not unhealthy. There can be subtle differences in birds of the same species, including the length of their crest or even tail feathers. A young bird will often have a shorter crest, but it is most likely to just be the result of a slight genetic difference.

A crest may also be affected during a routine moult.