I think we need to add it as there’s the option to upload fonts in Nextcloud so Collabora should use them.
This one should work but it will be overwritten by next update, it could be done with a systemd override like here to make it persistent but I think it’s better to add the option to the app by default.
@stephdl What do you think? Should I open an issue and a PR?
EDIT:
It’s not that simple, collabora doesn’t know about the nextcloud FQDN…maybe we would need another dropdown field in the collabora settings to choose the right NC instance for the fonts. 
Enable Nextcloud uploaded fonts in Collabora
Enter collabora:
runagent -m collabora1
Edit the collabora service:
systemctl --user edit --full collabora
At line 18 add the extra param --o:remote_font_config.url=https://nextcloud.yourdomain.tld/apps/richdocuments/settings/fonts.json
It should look like
--env "extra_params=--o:ssl.enable=false --o:ssl.termination=true --o:security.capabilities=false --o:remote_font_config.url=https://nextcloud.yourdomain.tld/apps/richdocuments/settings/fonts.json" \
Restart collabora:
systemctl --user restart collabora
Now the uploaded fonts should be usable in Collabora in Nextcloud.