If you have scored very high in the UCEED exam then it doesn't matter if you have less or average marks in the 12th exam.

Irrespective of your low 12th score, if you have secured a very good rank then it means that you have put in a lot of efforts and hard-work in the preparation so don't think about your scores now you cannot change that,just focus on what lies in front of you i.e UCEED exam.