Hey Aaditi

12th marks are given 5-10% weightage for your overall CAT score, that will affect the score a bit. This can not be predicted in advance about your selection chances of getting top IIM like Ahmadabad, Bangalore, Calcutta may not be sure but this may change and your 12th scores might get compensated by your Interview/WAT and CAT scores. There are institutes that have kept separate terms for candidates's 12th and Graduation like IIM-Ahmadabad have set a certain limit for scores in the respective level.

Also keeping a great CAT score may fetch you baby IIMs' and other good Management Institutes.

For further doubts, ask us. All the best