It is seen that usually, the exam conducted by ISRO, that is ICRB, takes students from the different branches of engineering that include electrical, electronics, and mechanical sections. So the exam syllabus is mostly based on these branches only.
Lastly, we can conclude that you have two options if you dream to work in ISRO. Firstly you can apply for a mechanical branch while filling an application for the exam or you can pursue higher studies and then apply for the vacancies in ISRO. This approach can help you immensely in turning your dream into reality.