Hi

Yes , 12th board examination marks are important

This is because,

to be eligible for admission in NITs, IITs  etc one needs a minimum of 75% marks in class 12 or equivalent exam or must be in the top 20 percentile in  class 12 examination. For those who are from SC/ ST category, they need a minimum of 65% marks in class 12 or equivalent.

Thankyou

For admission to NIT/IIT/CFTI through Jee Mains, you need to score at least 75% marks in class 12 or equivalent exam or they must be in top 20 percentile of your class 12 examination. 5% relaxation in marks for SC/ST/PWD candidates. If you're failed to meet this criteria, you'll not be eligible for admission to this universities.

https://engineering.careers360.com/articles/jee-main-eligibility-criteria