Ponchos are fine. Technically,
Boba Fett's cape was a poncho of sort and could be attached and worn as such. It's shown like that in pre-production images and in a lot of concept art.
Jengo Fett apparently wore an actual poncho and is the picture for the "poncho" entry on Wookieepedia.
But we all know the Fetts are a bunch of pretenders, right? Regardless of that, ponchos exist in the Star Wars universe, so you can wear one. I think where you might run into trouble though is if you try to get, say, a traditional Mexican textile poncho. Those patterns and colors won't fit.