Hey,

After doing diploma, you are still not a graduate. You are junior engineer after diploma. Now about scores, so even if you have low scores, you can get admission in various government and private institutes. You may have to give lateral entrance test and qualify it for eligibility.
After diploma in mechanical engineering, you can go for B.E or B.Tech in mechanical engineering, aeronautical engineering, instrumentation engineering, etc. You can also go for certificate courses like robotics, CNC, mechatronics, nano technology, etc.

Hope this helps!