Seeing as no one else is pitching in, I'll offer my anecdotal 2 cents:

Haven't stayed at the Moxy, but I've done my fair share of airport stays in Mascot. The Moxy is next to McDonald's, and while it's a reasonably easy trek to the domestic terminal, staying there for anything other than an overnight is a pain. The immediate area is still undergoing important upgrades so it's not walking (or roller-bag) friendly. It's an annoyingly long walk to Mascot station (15-20 mins with long waits at traffic lights) and the area has zero interesting F&B options (unless you're into fast food or airport hotel fare).

You haven't explained what you're doing after you're meeting your "someone" at 6:30am (staying in Sydney CBD, catching a flight the same morning?), if this is your first time in town, etc. So I can't offer more tailored advice. If your objective is just killing two days near the airport on the cheap, I'd say go for it, but if you could be instead exploring Sydney I would rather ditch the pricey international chains and get a budget friendly option nearer the CBD.