Quite different in niche for me personally:It seems so similar to the wombat in the niche it would fill in game.
- They're more of a filler species than wombats are, at least in Australia. We currently don't really have any useful filler species for Australia (closest is probably the wallaby) so they'd be very useful in that regard
- Quokkas are often kept in mixed habitats with other species while wombats normally must be kept alone. Ignoring the fact that they're found on the other side of the continent they're probably the best species we currently have to realistically mix with koalas for example (small mammals like potoroos, echidnas and quokkas will often be added to koala habitats to increase the amount of activity)
- Quokkas are not only able to be walkthrough as you said, but they're often in children's zoos due to their cuddly appearance and friendly disposition