One theory is that she was a bit odd-looking, but pointing that out would likely result in a painful death, so everybody erred on the side of flattery.
This is basically how half my women ancestors got husbands, as a result I look like someone tried recreating a neanderthal and ended up with a Norse-Gael.
Yeah, it is the exact opposite of what OP claims. Historians are basically the only ones constantly demystifying her, while her legend as a sexy femme fatale lives on in the general public mind (thanks pretty much exclusively to Hollywood, because let's face it, noone would know she existed without them).
Cesar sang praises to her mouth, which could be interpreted to be about her actual mouth, her oratory skills, or her oral skills. Knowing how Romans loved innuendo, probably both.