Is it good to pursue post-graduation in tool design from NTTF, Bangalore after B.Tech in mechanical? Does it have good opportunities?

Yes, this is a good choice. NTTF is a good college and in future there is a good chance to get opportunity in your field.

Pertaining to education the higher the better. Post-graduation is a benefit which you will find when you stand among the crowd. As far as opportunities are concerned, these courses exist because there is work in this field, else they would have been extinct by now. All the best.