does 74% aggregate in 12 class can harm placements and salary??? does 12th marks play a crucial role during placement??

As per what percentage of marks you have given overall in your 10+2 there won't be any harm to your future placements as most of the companies criteria falls under 60 to 70% only so I don't really think you need to worry at all and should concentrate on studying as things will fall automatically in it's place.

Best Of Luck!