One can easily compensate for poor past academics by scoring extraordinarily well in CAT. Anything below 99 is not acceptable if your past academics are already below average. Moreover, MDI does not give much importance to past academics while shortlisting candidates. They give maximum preference to CAT scores. From what I heard, the interviews are very informal and friendly. Not many formal questions are asked so one can be their true self inside and portray their talents.
However, you should be able to justify your low scores in the later stage.
So focus solely on your CAT preparation with MDI as your target.