I would have said joint responsibility between the flight crew and the ground staff.
I would suspect it was getting guided in by an electronic system. As such the pilot was probably focused on this rather than observing the area around the aircraft.
As such yes the commander is responsible and should have stopped the taxi. If in doubt, stop and think.
However you can't attribute their blame without accepting that someone left the ULD there (and it certainly wasn't the flight crew), in a area where it poses a threat and shouldn't be in the first place - both unsafe practises.
Working in a ramp maintenance environment my understanding / belief is that safety is everyone's business and as such, blame can't be shifted because 'I wasn't the last one to touch it' or 'someone else should have seen it'.