Toilet, which was pronounced with a fancy french accent in the past, would cover everything from the room, to the ass, to what's produced. Putting all those meanings in one word was supposed to cover up the process a bit, give it a bit of class and discretion.
I believe the title can thus be translated to "The Ass of a Sex Goddess".
It's from the French toilette, which originally meant "dressing room" from toile, meaning cloth. It's used as a euphemism like "restroom" or "washroom" but also still means "getting dressed routine." Eau de toilette (English: toilet water) doesn't mean water from the shitter, it means perfume.
So neither the English nor Spanish Wikipedia pages mention it, but La Venus del espejo can also mean Venus of/from the mirror referring to her appearance in said mirror.
No idea where they got toilet from. It couldn't be a translation mistake as neither word is anywhere similar to one another
Her role encompasses aspects of sexuality and fertility, certainly, but to define her as a “cumslut” would be an oversimplification of her significance in Roman culture.