Hello sanjoy,

I hope you are well.

Indeed your class 12th marks are very less but that doest represent your skills and abilities.What decides is how you will make yourself well rounded in MCA, the more you will develop you skills the more your chances get better for a good job.
At the time of placement they wont ask you about your 12th marks, what they will ask is how much you know about your current study and how skilled are you.
So relax and study well and you will surely get placed.

I hope this helps.

Good luck!