IIM's do consider past academics and so its not just the graduation performance that matters but your 10th and 12th scores as well.
Otherwise BMM graduate student is just like another graduate candidate and there is as such no special consideration.

To get into top bschools in general, overall, aim getting a percentile above 98 and a good profile.
Many b-schools judge you on a your profile and there you should have an impressive one.

It will be just like your bio-data, with your skills, hobbies, participation and organisational skills. How much you were involved in social and extra curricular activities.

Also your GD-PI.WAT rounds are important to as its like direct interaction with the panel. Through this you can not only form an impression but prove your worthiness if not performed well in the entrance. The selection parameter in each college varies and so for an example you can check the admission process for top 3 colleges below:

IIMA

IIM C

IIM B

Go through the above links and understand how the selection differs in each institute.
Good Luck.

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