All diseases are in a limited sense 'mental diseases' insofar as they are all potentially liable to induce various psychiatric sequelae relating to conditions like traumatic distress and anxiety that have to be treated and managed.
Good epidemiology works from this basis to identify the impact of disease upon a given population in both physical and mental terms.
DSM-5 defines mental disorder as "a syndrome characterized by clinically significant disturbance in an individual’s cognition, emotion regulation, or behavior that REFLECTS a dysfunction in the psychological, biological, or developmental processes underlying mental functioning". So, from my perspective, the answer to your question would be no.
Nevertheless, we could consider the psychological consequences of having become ill with COVID-19 as a mental disorder (not COVID-19 per se). We actually have a diagnosis for this condition: Adjustment disorder.
Adam Taylor Thank you for your observation. Then how do you relate when someone is unhappy in dream and that emotion affects on waking state?
Luis A. Centeno-Gándara Thank you once again. I agree with you. But apart from this pandemic how do you consider when someone badly affected by emotions in dream ?