Can a pole at the origin be considered a dominant pole?
You could get away with defining it as such (or extending the concept to the pole with the most positive real part).

But typically the concept of a dominant pole is applied to systems that are truly stable, and thus actually settle out.  A system with singular poles on the imaginary axis (and no poles in the right-half plane) is considered "metastable", because with the right input it has a bounded response, but with the wrong input its response is unbounded.