The number one thing Jetstar need to work on is there customer service when things go wrong. If everything is going well then their service is actually very good. I cannot fault their onboard service, the crew are always polite and friendly and I think their onboard menu is better than VA’s menu. Their staff in Manila call centre are unfailingly polite and friendly, and are generally very helpful where they can be. The problem is that they don’t have the authority to handle situations when things go wrong. The same also applies to their airport staff (although they’re contractors from Swissport etc.) who can’t do much of anything when there are delays or cancellations. This is what gives Jetstar a bad name more than anything else.
The other thing they need to do is consider their approach to maintenance. To be clear, I have no doubt whatsoever that they comply with all CASA requirements, and I’m categorically not suggesting they are unsafe. That said, they fly their fleet hard, and they need to be focussing on preventative maintenance to keep that up. There is only so much you can defer or MEL before aircraft need to be taken out of service, and then their entire operation falls over. I flew them twice last month and on both times we were delayed while engineering staff shuffled paper work in relation to technical faults that they didn’t fix. It’s great that they keep things moving on the short term, as the delays were only about 30 minutes, but these deferred items start snowballing until they can’t defer any more, then you have major delays or cancellations as they don’t have the slack to absorb this.
Back when Continental and United merged, they had major issues for a while with the reliability of their A319/A320 fleet. Continental had higher fleet utilisation than United pre-merger, and Continental had therefore invested in a preventative maintenance regime to get in front of issues before they arose. Pre-merger United maintained their aircraft in line with everything required by the FAA, but didn’t go much beyond that as they didn’t need to, they had the slack in the fleet to fix issues as they arose. Post-merger, United tried to fly the ex-United aircraft at the same utilisation as the ex-Continental aircraft, and the operation fell apart because they didn’t have those processes in place for the Airbus fleet. Once they realised that was the issue, and brought the maintenance on the Airbus fleet into line with the 737 fleet, the issues went away and dispatch reliability of both types is roughly the same for United. The point is that you can’t run a reliable airline on an oily rag.
I wouldn't frame the issue as such since someone with a predetermined view on Jetstar may view this as suggestion of substandard maintenance, which is simply not the case. I would also take umbrage with the talk of preventative maintenance since I'd argue that it's not really a thing in aircraft maintenance. All aircraft have regular service intervals based on various parameters including flight hours, cycles, days, etc for line maintenance (A checks, ST checks (B787 doesn't have a B check), C and D checks, etc). I have no doubt Jetstar complies with these. However, no rational airline would do any more than this. The question can be asked though if Jetstar leave enough time in the schedule to do these checks and if they do not, it's likely causing delays. Additionally, their maintenance capacity is limited, and the major issues they have had with some aircraft (e.g. VH-VKL's lightening strike) may have had the effect of sucking in maintenance resources while also being an aircraft short for some time (it can be a double hit).
In respect of the MEL, the MEL has limits and has to be approved by CASA. MEL's have categories with fixed time frames to be fixed within (see the CASA advisory CAAP 37-01v5.1). They can't just fly around with MELs indefinitely, and even then MEL items can preclude or limit operations on certain routes or at certain times. The purpose of MELs are ultimately to allow for things to be fixed during regularly scheduled maintenance, to keep aircraft flying while awaiting parts, or at a very basic level for the aircraft to return to base where parts, equipment and more resources are available. Knowing CASA's conservative approach I have no doubt that they would ping Jetstar severely if they were exploiting the MEL process and procedure to avoid and/or delay maintenance.
So I doubt that they are short changing on maintenance, however they are likely pushing the fleet too hard and over scheduling it, meaning that the aircraft are perennially being released for service late and thus generating delays. So it's not their approach to maintenance, but possibly their approach to scheduling. Secondarily, simply not having the manpower available to do that maintenance quickly enough. For example, a B787 A check will take about 100 man hours, but could take anywhere between 6 and 18 actual hours depending on how much (and the correct) manpower is applied to it. They may be over scheduling the aircraft relative to their maintenance manpower.