Is grabbing the dreadlocks from the back in NFL a penalty?

i think so, because in AFL, Chance Bateman got one of his dreadlocks ripped out.

my best guess would be, yes

the answer is no because the NFL has a policy, it is your own choice to wear your hair out of your helmet and the player defending is allowed to grab any part of your uniform other than the inside of your pads or facemask.