In top IIMS 10th ,12th marks considered and have weightage in final selection your total marks is taken for the admission. It gives a huge advantage to the students who performed well in the exams. 12th marks is really important in old IIMS like AHMEDABAD, BANGALORE,FMS,LUCKNOW, INDORE. They have weightage of these marks for admission.

don't worry if you don't get good marks as some IIMS like CALCUTTA and new IIMS give chance to students who dont have good academic record  and it just have few marks in selection criteria you need to focus on ECA or WORK EXPERIENCE to get your seat in final selection.