A hat is bought on proportion against a head and a face. The product photograph shows the hat. Whether that brim is too wide for someone's shoulders, or the crown too tall for their face, is not in it.
What the try-on shows
Proportion, mainly: how the brim reads against the shopper's face width, how much height the crown adds, how the whole thing balances. That is most of the hesitation in the category.
The hair problem
A hat interacts with hair — it flattens it, covers some of it, changes where the rest sits. That is a harder operation than putting a garment on a torso, because it modifies something already in the photograph rather than adding to it.
Expect the placement and proportion to be right and the hairline transition to be the weakest part of the image. Short hair and hair worn back give cleaner results than long or voluminous styles.
We have not benchmarked this category separately. The pipeline is built around garments; hats run through it and the mechanism is sound, but a measured pass rate is not something we have.
What helps
Face-forward, straight-on photographs, and a clean product image of the hat shot from the front. A three-quarter product shot of a hat gives the model an ambiguous read on brim width, which is the dimension that matters most.