No, your 12th marks doesn't matter after MBBS. The minimum eligibility for NEET PG is Candidates who possess MBBS degree or a certificate which declares the candidate has the provisional MBBS degree can apply for the NEET PG The certificate or the MBBS degree must be recognized by the Medical Council of India.

NEET is the entrance examination for admission to MBBS,BDS, Ayush and Veterinary courses across India. So to be eligible for NEET, you should have completed your 12th, passed in all subjects and secure a minimum aggregate of 50% in Physics, Chemistry and Biology. It is 50% for General Category, 45% for PWD and 40% for OBC,SC,ST. Now if you belong to OBC Category, the Category you belong to should be also listed in the Central OBC list. Or else you will be considered as General Category and so you have to secure a minimum aggregate of 50% in Physics, Chemistry and Biology. This 50% is inclusive of the Theory and Practical marks scored in Physics, Chemistry and Biology collectively. You needn't score 50% in Physics, 50% in Chemistry and 50% in Biology separately. You need an aggregate of 50% or 40% depending on your Category as an aggregate in Physics, Chemistry and Biology. And along with Physics, Chemistry and Biology, you should also have studied English as one of the core subjects in your 12th. For PG, USMLE it is not the 12th marks that are counted. And most competitive exams only consider Graduation and not 12th.