Except that Garnet didn't really beat her in a one-on-one fight. Yeah, Garnet was outmaneuvering her but nothing she was doing was really doing damage to Jasper. Even suckering Jasper into flying into the engine and crashing the ship into the Earth FROM ORBIT didn't do a whole lot more than piss her off. Hell, in future appearances when Pearl and Garnet go out to look for Jasper, I'm pretty sure they planned on fusing into Sardonyx to beat her.
However, with subsequent appearances, much like Peridot and Lapis, she suffered from some rather painful Badass Decay. Although at least unlike the other two, she at least got some character development in the bargain.