Does Jetstar cancel flight with lower loads? Yes.
This only really occurs if the
schedule let it work, ie-most point to point sectors, not those via another virtual base then off to somewhere else before making its way back to the start. For instance, I flew a SYD-AVV flight on a Tuesday evening a few weeks back, and did this on purpose as I knew the previous flight which was 1hr earlier would be canceled (too much competition for a Tuesday to even cope now on this route). Mainly due to the last flight being a Avalon based A320 and the previous flight was a Avalon return I knew that it would still go ahead, even with the 30 pax that accompanied me. In fact, I would say the majority of Jetstar's cancellations lie on Avalon routes or to Coolangatta from Sydney.
And here we go, Jetstar's own Simone Pregellio even states it:
A Jetstar spokeswoman said flights were sometimes cancelled for scheduling reasons if they could not be filled but that it was not a daily occurrence.