Is insecurity a sign of mental illness?

Not unless severe: "mental illness" is a judgment call. Severe mental conditions, e.g., schizophrenia, are clearly illnesses, while moderately bad depression is debatable, and most would agree mild to moderate insecurity is not. But if a person has severe insecurity that interferes with everyday life, it might be considered an illness. Whether it's an illness or not, counselors & therapists can help.