Currently the Incus default profile includes a definition for an ethernet connection, but since the UI doesn’t provide any way of modifying, creating new or changing the profiles for VMs, it’s not possible to e.g. create a VM without network connection without dropping down to CLI.
Personally, I don’t think the default profile should even include a NIC definition or the UI should take the NIC from the default profile into account and display it and allow configuring it. At the moment, the UI doesn’t show even a hint that all VMs and containers do, actually, have a NIC included as the NIC box is empty – this is contradictory. Alas, that’s a separate issue and kind of dependent on whether they decide to do something about the profiles or not.