I have never seen a subtitle in Plex that uses the Courier font face. (except when it was a pixel-based subtitle format)
Which type of Plex app are you using and what is the platform where your server is running on?
Courier sounds like the last fall-back when there is no access to any font resorce at all – i.e. either an issue with the server (when subtitles are “burned in” during transcoding) or with the client.
Btw. this guide is still working and it definitely does not produce subtitles with the Courier font face: Customizing Subtitle appearance in Plex Player for Windows/Mac/Linux and Plex HTPC and PMP
