Could I Do a Fat Transfer to Make my Forehead Not Look Slanted.

Improvement in the slope of the forehead is not best done by fat grafting as it will create irregularities and unpredictability of fat survival. Forehead augmentation done through the application of bone cements through an open approach is very effective and will create a smooth and less angulated forehead shape.

The forehead problem you describe is known as oxycephaly and is due to the premature closure of on of the cranial sutures(metopic) during infancy. As a general rule of thumb you cannot use fat grafting to effect large changes in contour as the grafts don't really have the structural integrity that would be needed to contour an entire forehead. Forehead contouring is rather complicated but as a rule must be done by manipulation of the bones while protecting the frontal sinus that lies within the forehead bone. Consult with a Board Certified Plastic Surgeon with experience in Craniofacial Surgery.