As you have done Diploma after 12th then you need to show the 12th certificate. Now if you don't declare your 12th then it can happen but in that case you need to say lie to the board. So its your decision. But if you stated that you have passed 12th then you aren't eligible for it.

Now its a point you should consider based on your intelligence.

I hope this answer helps. All the very best for your future endeavors!