this year cut off rank will be increases or decreases 2020 jee mains For NIT and IIIT's?

See, cutoff of NITs, IIITs, GFTIs and IITs depends upon vacancy of seats of these colleges. You won't observe too much fluctuations in these cutoff ranks because each year the number of seats in these colleges are constant. In the year 2018 there was a fluctuation because supernumerary seats were added up for female candidates. Also in 2019 we observed fluctuation because a new category was introduced named Economically Weaker Section ( EWS ). But this year we observed pandemic and though many students could not appear in exam the ranks vs percentile statistics did not vary much as compared to previous year.

So better not to expect the fluctuations of cutoff ranks. The first round of allocation will be conducted on October 17 and you will come to know how much fluctuation there is compared to previous years.

Good Luck.