Within the airport and all the way until the airport train station, there are elevators (also to the underground platforms). I have no clue about how far the airline’s service extends, but I think it should be doable even without assistance for somebody who needs a rollator.
Most of the trains SBB uses on the IC81 line (A direct train from Zürich Flughaven / Zurich Airport to Interlaken Ost) have step-free access in at least some of the carriages. As the airport station lies in a curve, the gap between the train and the platform edge can horizontally still be relatively big (I would guess around 20 cm) and not all trains have an extendable board to close the gap.
A few older trains or carriages are not accessible without climbing a set of stairs.
In order to get more precise information, you can call the handicap information centre: +41 800 007 102 from abroad.