Do I need to have some work experience to get placed in a good firm after completing MBA from a reputed B-school?

No, it's certainly not required. But, yes, if you have any then it will work as an icing on the cake otherwise your merits can take you there.

It's not mandatory that an individual should have work ex. to get placed in campus placements. Companies do recruit freshers. Companies just look for the analytical, reasoning, decision making and managing skills. One should give smart answers to crack the interview.