What are the prospects of doing MBA after MBBS?

This would depend on your interest areas and career aspirations. In case you intend to pursue a career in medical practice / research / academics area then MS or PG Diplomas in related field is better. However, if you want to move into general management the MBA is better.

These are two different fields, better to go for MS or PG in same field.

MBBS and MBA are quite vast fields. MBBS will make you pursue medicine and MBA will help you understand business management.