I don't like the straw boater hats. I have a few straw hats and know how straw looks. A boater hat should have a flat brim. Image Searches agree with me.

These straw boater hats do not look like straw boater hats to me.

Maybe part of it is the angle, but they look like they have flat brims to me? I'm not an art expert so I can't comment as well on perspective and all that, but I could tell what they were going for when I first saw them anyway.

I agree with Laurey, the curve is likely because they are tilted backwards at an angle. If they were completely straight they would look weird. Also, the straw is a bit simplified but I think adding in extra lines/details to represent the straw would make it look too crowded.

I like them a lot :3 I think they fit the angle of the HA very well, which is tbh kind of awkward to work with

I can definitely see where you're coming from, with them not being totally flat. They give off a round vibe, maybe it's just this artists style. They're cute but I admit they're not an item I'd use a lot of, but they do have a nice summer vibe!

It might be this part that's making it look curved

Maybe it should follow the head's curvature?

Alternatively:

I don't like these at all... And I don't think they're worth improving :')