The main reason for them being kittens is to incentivize valuing the astronauts lives without forcing a gameplay related penalty for killing or losing them. The disposability of astronauts is something the devs of this game think was a mistake in KSP. But at the same it's not a game about forcing the player to behave a certain way, so making them kittens is the middle ground.

What disposability? Kerbals were never disposable! If you crash your little green astronauts on some moon or planet, you're supposed to send a rescue mission after them. And should that rescue fail too, stranding more Kerbals, you just keep launching more rescue missions, until you successfully establish a colony :).