I'm going to wager it's too tight... Adam Pawlus mentioned one figure's being too tight to fit, but I'd guess the helmets are almost all re-used from other figures (most likely the removable helmet Stormtrooper we got in the last couple years there).
That helmet's designed to fit a small, smooth-headed, cowl wearing character... Some of Joker Squad, if not most, seem to have a boofant of hair... That's a problem if the helmet's not made to fit all that bulky hair-did.