I don’t think the robes were meant to be specifically Tatooine farmer outfits, but rather farmer outfits in general. In a lot of pre-PT material they’re referred to as “colonist clothing”, which makes sense considering how humans aren’t meant to be native to Tatooine.
(It’s hard to say since this was never addressed in the movies and that a lot of this info comes from EU material, but I don’t think it was ever blatantly meant to be just Tatooine outfits. At least not post-ESB.)