Without atmosphere, capes wouldn't billow aesthetically. I'm not sure what they'd end up looking like, but I imagine they'd awkwardly just sort of hang there at a weird angle, and swing with the wearer's rotational movements like a flexible pendulum, getting in the way while looking dorky.
Flags also had support structure so that they stand nicely. Some servos could easily make elegant waves.
just shoot a little bit of the suit's air out of your butt at the cape
The Incredibles taught me why you wouldn't want capes[1], especially in space and rocketry.