As a Jetstar customer you are entitled to use the lounge. I booked a Qantas flight through the Qantas website which was on a Jetstar aircraft and some $70 more for the same flight.
Hobart specifically now closes their lounge at 5pm, presumably as a cost-cutting measure.
What also happened recently was my Qantas flight was cancelled and intended up on a Jetstar flight that cost half as much, but alas the lounge was open late in Devonport.
It's all in the terms and conditions. Interestingly Qantas mainline only fly to Honolulu once a week, however they still open the lounge for the more regular Jetstar flights.
They still serve a hot dinner in economy particularly on city-flyer flights. The flight was levelled with 'dinner' but they served 'supper' plus they charged for alcohol. I've never had a cold sandwich before, but then again this wasn't a city-flyer flight.