Hey,
Don't worry.
According to the previous rules and regulations, each and every candidate had to obtain a minimum of 75 percentage marks in class 12 or equivalent exam or they must had to manage themselves among the top 20 percentile students in their respective state to get admitted in any NIT.

But due to pandemic period, HRD minister Ramesh Pokhriyal Nishank decided to make this eligibility criteria easier to increase the chances of admission in all of the topmost and esteemed NITs and has declared the " relaxation of JEE MAIN 2020 eligibility criteria for NITs and CFTIs " on 23rd June,2020.
According to this, a JEE Main qualified aspirant has to obtain just a passing certificate in class 12th Board examinations irrespective of marks secured.
For further details:
https://news.careers360.com/jee-main-2020-result-eligibility-for-admission-nits-iiits-and-cftis
Hence I can assure you that your low marks at Mathematics in your Board examination can't affect your admission chances of top NITs. You can get instituions only as per your secured marks and rank in JEE MAIN 2020.
So don't need to be worried. Just relaxed. You can also use Careers360 JEE MAIN college predictor tool in this regard:
https://engineering.careers360.com/jee-main-college-predictor?icn=QnA&ici=qna_answer
If you still have any query regarding this, you can ask in comment section.
Wish you all the best for your betterment in future.

Hi there,

Yes since your percentage is more than 75% therefore you are eligible for nits.

By the way this year even this percentage is not applicable, so it is 100% sure that you can get nit if your rank is good enough for it.

Here is the college predictor you can check your options here

https://engineering.careers360.com/jee-main-college-predictor?icn=QnA&ici=qna_answer