Lemmy instance requieres at minimun 150mb of RAM, the CPU usage is minimum. Don't expect it to scale well to a lot of users, but it should work for a few users. More info here: https://join-lemmy.org/docs/en/administration/administration.html:

Running one on a 6$/month droplet. The TLDR is you'll definitely need a swapfile for spikes, but for a single user it'll probably work, though be a little slow to load communities for the first time. Mine is subbed to a huge amount of communities and so far the disk usage is nothing serious

You won't be able to host a big instance.

According to this thread https://lemmy.ml/post/440678?scrollToComments=true 1GB of ram should be enough for a small one. The pi zero has 512MB. I'd say you should still try, just so we know

No, the pi zero is not going to be capable of running a Lemmy instance. I wouldn't recommend running it on a Pi at all unless you plan to store files on an SSD. I currently run my very tiny instance with just two users on a vpc with two CPU cores and 4gb of RAM but I still occasionally have performance issues.