if in jee mains cut off goes low than it will be difficult to take admission or not means cut off of colleges will close on low ranks or not

No it's not necessary that cutoffs for JEE Main has gone low so the college cutoff will go low. Actually the JEE Main cutoff has gone low because the JEE Main board has to qualify 2.2 lakh candidates for JEE Advanced. So the requisite number of candidates have to be qualified. Since the 2.2 lakh students have secured marks above the cutoff, that is why the cutoff has been decided as such. The college cutoffs will roughly hover around last year cutoffs only.