score:3
I've done a similar transfer in the other direction recently. The Domestic and International wings of the airport are only a 10-min walk apart. If you check in to both flights in advance, with electronic boarding passes, and have no bags, the connection will be straightforward. However, DPS-SIN is an international flight, so you will need to clear Immigration at Denpasar, and depending on your nationality (in particular, if you need a visa for Singapore) Jetstar may require you to check in in person to show your documents.
My main concern with your plan is that Air Asia does not have a great on-time record, so you don't have a whole lot of buffer if they're delayed, and Jetstar will show you no mercy if they are. A direct flight from Lombok to Singapore would be safer and much more convenient.