Hello,
As per eligibility criteria for Bachelor of Art (BA) that is  a candidate should pass 12th standard. As you are a diploma holder, you don't satisfy the basic criteria for that.

As you have completed your diploma in engineering then going to Art side can be confusing decision, you can go for b. tech  after your diploma course.
You can check the eligibility criteria for Bachelor of Art by going through this link :-
https://www.careers360.com/courses/b-a-bachelor-of-arts

Also to know opportunities after diploma of Mechanical engineering go through this link :-
https://www.careers360.com/courses/diploma-in-mechanical-engineering

All the best!!