How to make a passion fruit mousse without gelatin?
There are two things you can do to make the mousse stiffer: 


Reduce the water from the fruit. So use some kind of concentrate instead of the pure fruit. For example, you could cook down a syrup or jam and add it to the mousse. Or see if dehydrating juice gets you somewhere. 
Use more fat. Instead of whipping cream at 30 to 35% fat, you could use double-cream at 45%, or a mix of cream and mascarpone. Or fold whipped cream (for aeration) and some cooked down passion fruit into something even stiffer. Maybe base it off a whipped white chocolate ganache.