Is it OK to show postnominals differently abbreviation for that degree eg MCC instead of CCM?

No, it's generally not okay to show postnominals using a different abbreviation than the standard one for that degree. Using "MCC" instead of "CCM" (assuming CCM refers to a Certified Clinical Medical Assistant or a similar credential) is incorrect and could be misleading. Professional credentials and degrees have established abbreviations, and deviating from them can create confusion and cast doubt on the validity of the claimed qualification. Always use the officially recognized abbreviation.