Hello

Most of the MNCs and other IT companies do not care about % of marks obtained by you in your 12th examination but few companies does. So, yes , your class 12th marks may affect your placement after your btech .
Although class 12th marks doesn't play much role in placements and they mainly help you to get into college but now a days companies wants you to have at least 60 % marks if you want to seat in their placement process and anything less than 60 % in 12th will make you ineligible for the placement process

So , you should try to get good marks in your 12th examination, as it's always better to be on the safer side.

Thank you