After MBBS, doing M Sc psychology means adding an additional degree to your portfolio. That does not hurt. But what is the goal?
An MBBS can treat psychiatric patients, but a psychologist is not licensed to manage psychiatric disorders.
Thus, if the intention is to practice psychiatry, an M Sc gives you additional advantage of understanding psychology of the patients better and also see patients for psychology counseling which otherwise a psychologist at your practice would be doing.
Not wholly comparable, but It is akin to going to a dental practice. Typically, a dental hygienist would clean the teeth of the patient. But if the dental hygienist is on leave or unavailable, a dentist can do the job. But vice versa is not true.