In colleges like IIM Ahmedabad, its always the Competition between the best. Hence, though there might be exceptions or outliers, its always preferred you have a 85-90+ Grades in both 10th and 12th as well as a 8-8.5+ CGPA in your Respective University. Definitely this doesnt mean, youre out of Competition if youre Profile doesnt match with the Criteria, you can always make up with a Great CAT Score. However good grades in your 10/12/Graduation reflect your strength in Academics throughout School and Colleges and does add up to your profile be it in Calculation of Net Score or for a First Impression in eyes of interviewers.

For more information about IIMA Criteria and Cutoffs do follow the link below

https://www.careers360.com/university/indian-institute-of-management-ahmedabad