Hi Anshu!

You may or may not have a problem with these marks. It all depends on which IIM you want to get into as each IIM has its own criteria. 

You can get all the required information about the priority given to 12th standard marks in each IIM in the following link:

https://bschool.careers360.com/articles/iim-admission-criteria-2018-20 

But either way, dropping for 12th standard doesn't seem to be a good idea to me.

Hope it helped you.

All the best!