There seems to be no way to set the nick name in an XMPP multi-user chat (at least not in the UI). Instead, KTP always choses the user name of the XMPP address (for example if i have xy@example.com as an XMPP address, the nick name will be xy). Reproducible: Always
Dear user, unfortunately Telepathy is no longer maintained. Please migrate to another solution, e.g. for Jabber a possibility is Kaidan, for Matrix a candidate is NeoChat.