sometimes women are given higher billing just to fool viewers into thinking there are some love scenes in the movie. Marianne Koch has a very small role in FOD yet was given higher billing than actors with much bigger roles. Billing is solely dependent on size of the role. There are many movies that have lots of men and one woman in a small role where the woman is given higher billing than she deserves.
Mara Krupp is revolting but IMO her character is meant for laughs more than anything. When you see how she prances for Monco (right in front of her husband), it's funny -- much funnier than if she had been a pretty, skinny girl.
Of course, that may not have been the original intent, since they shot the sex scene; which THANK GOD they did not include in the final cut. But as it is now, where she just have those two brief scenes, it's funny how this nasty woman with the enormous wobbly melons is prancing for Monco
On the commentary, Frayling says that Krupp was used cuz she bore a strong resemblance to the wife of a producer that Leone detested