I have completed my B.Tech. Can I apply for post graduation in mass communication?

Yes, it doesn't matter, either you are graduate with B.Tech or B.Com or BA only. If you are interested then you can take admission in PG course of mass communication in any Indian university. But before applying you have to check the eligibility criteria of universities for the same wherever you app

Yes, you can apply. Graduation course does not matter.

Yes, definitely you can go for mass communication after your B.Tech degree, and this course has a great scopes ahead and you can get good opportunities after completing this course.