Hello,
Yes an mbbs graduate from other institute other than AFMC can join Indian air force provided they clear the NEET pg entrance exam with the required cut off.
However Almost 60% pg seats are reserved for the afmc mbbs graduates so the cutoffs for the non reserved seats is pretty high than other colleges.

The following is the priorty list followed while allotment of seats:

1. Priority I - Currently serving officers of AFMS

2. Priority II - Foreign candidates who are sponsored by the Government of India. Such candidates will be accepted on the approval of Ministry of Defence/DGAFMS/Ministry of External Affairs

3. Priority III - Medical officers sponsored by Parliamentary Forces or other organisations of Government of India. For these candidates, it is mandatory that they furnish sponsorship certificates with subjects for sponsorship by the authority to be considered for counselling

4. Priority IV - This category is of officers who are ex-servicemen and have completed their contractual service Candidates in this category of priority can apply only if they have completed their contractual service on or after May 1, 2017 or before April 30, 2020. This does not apply for candidates who resigned/dismissed/removed from their services

5. Priority V - Civilian doctors who fulfil the eligibility criteria and are ready to serve AFMS for a duration of five years after having completed the course are eligible under this priority. Doctors under this category have to sign a bond of INR 29,00,000 to the Government of India for AFMS PG 2020 medical admission

For more information click on the link below: https://medicine.careers360.com/articles/afms