Is there a place that people share their configs?
You could ask on the NixOS Discourse? I'm sure people would be happy to share some fun stuff. I would, but I haven't fully removed all the secrets from my config yet :D
NixOS on smartphones?
I know nothing about this project, but some effort has seemingly been put into this idea already :)
Personally, the more devices I can use NixOS on, the happier I am.
Declaratively installed VM?
No, not by default. There's other people's work that lets you do this, but you're likely best off doing it yourself by generating qemu config files and systemd services for them through Nix.