Yes, if you appear for the CBSE improvement exam as a private candidate and score higher marks, then the latest marksheet will be considered as your valid Class 12th result. For IIM A, B, C and other top IIMs, they look at the final updated marksheet that you present at the time of admission.

So, if you improve your percentage from 70% to 90%, then definitely the improved score will be counted in your profile evaluation. Just make sure you give the exam through the official CBSE improvement route and keep the valid documents.