Tilted pelvis/vulva in horses?

No, it will not cause problems when the mare is ultrasounded. What you are describing is a pneumovagina. It can be due to conformation or if a mare has had many foals in her lifetime. The reason it potentially causes reproductive problems is because the vulva can form a sort of "shelf" under the anus. If it is severe enough, manure can be drug into the vagina and cause problems. Some mares also are able to suck air into the vagina (this is where the name comes from). If your mare was a breeding animal, and the pneumovagina was severe, the vet could put in a caslisk to keep the vulva mostly closed and keep air and manure out. In most mates, this is not necessary. Hope this helps!