Yes it is a general purpose WebDAV server and should work with Zotero. I have not tried it with Zotero specifically, but I use it with several other apps that only offer normal WebDAV support such as Floccus for bookmarks syncing.

Nextcloud uses Webdav internally, but especially the Nextcloud apps expect a lot of additional non-standard extensions to WebDAV so that they don’t work with normal WebDAV servers. KaraDAV has reverse-engineered most of these extensions and thus the official Nextcloud Android and Desktop apps accept it as if it was a real Nextcloud. It is hovever limited to the file sync feature of Nextcloud, no calendars and such.