I haven't used an FM-1, so I can't speak to the specifics of it, but you absolutely do not need to understand music theory and improvisation to make music on something like this.

Additionally, understanding theory and improvisation is certainly a great thing to know if you're trying to make music, but you can understand a ton of things in the abstract and also be pretty terrible at applying it in the moment.

So if you want to get a device you can throw some hacked firmware on and have some fun, I'd say don't let your lack of theory hold you back.