At least on my client system (Arch Linux), I can just fire up a local spice client with
incus console $remote:$container --type vga
for any remote where the incus API is reachable over HTTPS.
I just had a look at how incus does that, and apparently it forwards the spice stream (which it gets over the HTTP API) to a local unix socket, and passes that local socket to the spice client (in my case, that’s “spicy” aka “spice-client-gtk” which comes with the Gnome desktop environment).