I think it depends on the use case. The power of docker is that you don't need virtual machines with set memory reservations, so configuring applications to grow and shrink memory from a shared pool is a lot easier. Not requiring an extra kernel is also a positive because every kernel needs to do some busy work to maintain itself, no matter how little it is, so spawning a large amount of micro VMs will bring more overhead than spawning a large amount of docker containers. On the other hand, virtual machines are obviously a lot better for security.

The performance overhead is probably not the biggest reason why you'd stick with Docker, though. The real kicker is that most existing services already run Docker containers and there are loads of resources out there to get you started. Firecracker is relatively new and unsupported. There's no technical reason why Firecracker is a better use case for an RPi, but I think people running RPi servers are on average less experienced and will likely benefit from the Docker knowledge base out there compared to, say, people renting a VPS somewhere, or people building full server machines.