Yes, you are eligible for the UPSC exam after completing your BA, even though you did a diploma course after 10th standard.

The UPSC exam focuses on your graduation degree for eligibility. As long as you have a Bachelor's degree from a recognized university, your diploma qualification won't affect your eligibility.

A candidate must have a Bachelor's degree from a recognized university to be eligible for the UPSC Civil Services Examination (CSE). The UPSC doesn't specify the stream of your Bachelor's degree. Having a BA degree makes you eligible regardless of your major subject.

https://competition.careers360.com/exams/upsc-cse

I hope it helps!