It should be understood that while her looks surely played a part in her getting cast on a permanent basis, that wasn't how she got in the door. They didn't post an ad looking for female models/actresses. She was an intern on the show, working there because she loved creating and wanted to work for M5. She got called on camera to help with a myth (by providing a mold of her butt) and that was what started it.
Skill got her the position with the show initially,
Also as you note, personality goes a long way, and she has a very good one for the screen. That is why Adam Savage is a part of the show. Mythbusters was originally pitched to Jamie Hyneman but he knew, correctly, that he wouldn't be able to carry a show like that alone because of his dry personality. So he suggested Savage, who he'd worked with in the past, in part because he's a goofball.
With a show like that it takes a combination of skill and presentation to make it a hit, and that was what the hosts had, Byron included.