Since the certificate clearly states that it is a Diploma, then, yes, obviously it is a Diploma and not a Degree. As for the postgraduate aspect, yes, an MBA, that is, a Master of Business Administration is considered to be a postgraduate qualification.
Now the aspect that you need to consider is the institute from where you completed this diploma. An MBA Degree program, whether a fast-track one of one year or a regular one of two years, is generally offered by reputed B-Schools, and an MBA qualification through such programs is accepted by potential employers. An MBA Diploma from a lesser-known institute, on the other hand, may possibly not be preferred, especially if it is not a UGC-recognized one. So do check out if both, the institute where you studied, and the course which you completed, is a UGC-recognized one, by vising the UGC website and checking under the list of colleges. Refer- https://www.ugc.gov.in/.
When you make a commitment, you create hope. When you keep a commitment you create trust! ~ John C. Maxwell