... sillier than "yellow sun = superpowers"? Sillier than "bathing in toxic chemicals = immortality"? Sillier than "bat extract = kill-crazed were-creature"?
Zee speaks backwards because that's how she was taught. She was taught by a showman, who wanted an easy way to come up with words that "sounded magic" onstage. If she'd grown up with her mother, she might not have been a logomancer at all, and could have used a wand and hand gestures instead, just as Bruce could have learned drunken kickboxing* and krav maga in Israel instead of judo and tae kwon do in the far East. But logomancy gives Zee a distinct niche compared to the Dr Fates and Morgaine le Fays and Ravens, who just wave their arms and babble meaningless pseudolatin, and it ties in to her character as an entertainer.
* I'm pretty sure drunken kickboxing isn't a real discipline, but I'm even more sure that it should be.