Cape is a slang term that refer to people with powers (parahumans), and was inherited from the universal costume symbol of superheroes and supervillains - the cape. It applies to most of the parahumans in north america and is applied to other parahumans outside it. Regardless of how closely those parahumans fit into the "suprehero" paradigm.
A small note of irony is that relatively few parahumans actually wear capes. A hold over perhaps from the adapted culture of the comic books.
↑Eighty heroes, if my bugs were counting right. Maybe eight in all were in the air. It made it easy to find Eidolon. Like Grace, his costume had been tinted by the film of dried fluids. He was a few stories above the ground, and his cape flapped around him in the strong winds.
It was hard to make capes look good. They had a way of clinging to the body, or flowing the wrong way, getting caught around an arm… it took a measure of majesty to make it work. Eidolon could pull it off.
Ironic, that the slang for a parahuman was ‘cape’, and so few of us wore them.
I’d worn a short cape, not so long ago, barely long enough to reach the small of my back. I’d adopted it more for utility than style, to give me more concealed area to hide my bugs and for the marginal extra protection it afforded me. I didn’t have it now, and I was somewhat glad. I might have felt more self-conscious, seeing Eidolon up there. I’d wind up worrying if I really had the ability to make it look good, when I needed to focus on projecting confidence. - Excerpt from Scourge 19.5