With a gap of 3 years (no work experience, few certifications) and low academics (53% in XII and 6.1 CGPA in a BE), what problems can one face during summer placements at JBIMS (MMS Finance)?

The first 2 semesters comprise of General electives that are compulsory for everyone. You can at least stop stressing about your specialization for a year. You need to be extremely flexible in order to pursue an MBA. I have seen many people who initially were into Finance yet after summer placements chose Marketing.

As far as the problems related to your low scores are concerned, well yes, it will always be a hindering factor. But this does not mean your MBA is of no use. You will get offers from startups which are an altogether new experience as you get to learn from scratch. But this problem does not happen during Final placements because placements depend upon how you have lived your college life so far and gained from it. At this stage, your past academics will rarely matter. 

You will definitely get a good job if you scored well throughout college despite having completed your internship from a startup or not-so-popular brand. Work hard, build your profile, and make this MBA a turning point in your life. Inculcating GD and PI skills is very important to ensure a good stable job.

So you can overcome the little struggle during summer placements and wait for the long-run benefits.