According to popular perception, IIM Calcutta places a high value on CAT scores, so you can overcome your previous academic failures and gain admission to a tier 1 MBA college by working hard on the CAT.
The reality, however, is quite different. People who have done well in CAT are generally people who have done well in academics in the past, i.e. 10th and 12th grades. This is because the basics are retested in CAT if you put in the effort back then. So the only thing you can get away with in IIM C is your graduation score. However, some top companies use this to shortlist candidates for summer placements.