Thing is though it's still wrong. A mullet has to be cut short on top and the sides or it's not a mullet. Mullets aren't tied back, they don't have to be, because the top and sides are short and only the back is long. Maybe what he should have done is work in a scene where Dick takes down his hair for whatever reason, so that people could see it's long all over and just held back by the hair tie.
Calling Nightwing's hair a mullet is just as silly as saying Adrian Paul's ponytail in his role as Duncan MacLeod in Highlander: The Series was a mullet. It's just inaccurate.