If you look at the eligibility critiera for Delhi State Quota at https://medicine.careers360.com/articles/du-mbbs-admission it mentions that you should have completed your 12th in a school located in NCt of Delhi. If you fulfil that criteria, you can use your SC Category certificate for claiming SC reservation in Delhi State NEET Counselling. Those Candidates who have completed their 12th in Delhi are eligible for Institutional quota and so they do not need Domicile certificate. Your Category certificate is not a proof of domicile but by the virtue of studying your 12th in Delhi, you are eligible for its Institutional quota and do not need a Domicile certificate.