Yes it matters in case of IIM's and admissions through CAT.

In each IIM there in an individual selection process and therfore is most of the IIM's academic scores are a must thing and a strict aspect to judge.

Say in IIM A, If you have 90 percent above in your past academics say in 10th, 12th and even graduation then you can get 10/10 (scaling out of 10) and it shall differentiate in other IIM's.

Like in top 3 IIMS A,B and C, check the selection process below and know the weightage of the exam, GD/PI/WAT and past academic records in the link below:

IIM A Selection

IIM B Selection

IIM C Selection

Hope this gives you an idea to score what and in which manner.

All the best!