It’s certainly an interesting idea! I could definitely see some future where Joplin has plugins that allow for that sort of thing (I’m not a dev so take that with a grain of salt). That said, implementing a flash card system is a pretty far stretch for a notetaking app, so I would guess that this isn’t likely to come about soon.
There have been similar requests before notably this one where [@foxmax links to a tool they developed that integrates Joplin and anki.