Usually, sleepwalkers engage in action that reflect stressful events that occured during waking hours.

To answer the question, no, sleepwalking, unless frequent enough to put one at serious risk is not considered a mental illness and actaully affects a lot of people, mostly adolescents.

Sleepwalking incidents are normally short and not dangerous, although sometimes, self-injury happens among other things that are much more dangerous for the sleeper.

While stress can be one factor, researchers have not found a definitive cause for sleepwalking and is now assumed to be a mixture of both physiological, psychological as well as chemical interefence in the system.