I want a media server, but can it look like total crap please?

Tell us you don't have a wife without telling us.

I don't know about jellyfin, I believe in plex you can set your own Metadata source, so presumably you could point it at a blank source, or block the internet metadata sites at thw dns level if you wanted everything to look broken with just text and a file-extracted thumbnail as the coverart 🤷‍♀️