Is Tasmania worth making a separate trip?
Often, yes. Tasmania is compact by Australian standards, but Hobart, Freycinet, Cradle Mountain, Bruny Island and the west still need time, a car and weather margin.
Answer
Tasmania is smaller than the mainland, but it is not a quick island sample if nature is the reason to go. The roads are slower than the map suggests, weather changes plans, and the best days often involve walking, food, coast or mountain time rather than constant relocation. It can be a brilliant separate Australia trip or a strong add-on if you cut elsewhere. It works poorly as a rushed afterthought once Sydney, Melbourne and Queensland have already taken the oxygen.