No. Adjustment Disorder is an emotional response to a stressful event. It may be immediate or delayed up to three months of the onset of the stressor. It is most common among adolescents and common precipitating stressors are school problems, parental rejection divorce, and substance abuse. Among adults, common precipitating stressors are marital problems, divorce, moving to a new environment, and financial problems. Symptoms may not immediately subside with cessation of the stressor but should begin to diminish. Medications are often not needed for adjustment disorder although some patients may benefit from psychotherapy or family therapy.
Serious mental illness is a mental, behavioral, or emotional disorder which substantially interferes with or limits one or more major life activities. Find out more here: https://www.nimh.nih.gov/health/statistics/mental-illness.shtml