Some images. I asked this many years ago, and at the time I thought it might have been a ‘pancake hat,’ but I don’t think there was any consensus.
I’d call it a granny hat. 
It looks like what men would call a “driving cap” or hat, but only if it weren’t so obviously for a lady.
I have a friend who’s a professional costumer for theaters. I sent him a pic and asked. Will let you know when he replies.
It looks like different hats. I’m seeing a straw hat and a Kangol (classic) hat.
More discussion of ‘Beverly Hillbillies’ hats on this somewhat obscure website from 2005.
Jed wore a slouch hat most of the time. In his Sunday-go-t’-meetin’ clothes, it was a bowler.
Wow. . . That OP was really an idiot to not know the difference between a porkpie and a trilby. Mr. Drysdale wore a hat, 90% of the time it was a Homburg. The other times, it was a trilby, not a porkpie.
Very reminiscent of a hat style favored by President Eisenhower’s wife. They were called Mamie hats in her honor(?)
I’d call it a Watteau, like these: https://www.skinnerinc.com/auctions/2664M/lots/537
It’s a wide catagory, though, that seems to encompass a lot of shapes.
Gosh… Give the kid a break not everyone’s a hat maven like you.
Didn’t Granny occasionally wear a Confederate soldier’s cap? Especially when she felt she was going to war with someone?
This has been a Filmways presentation!
Well, that only took 11 years.
And well worth it. I did not realize they were a thing. Now I’m all about Watteaus.
…dahling!
Oh, sorry, wrong show!
Choose: Elly May Clampett or Lisa Douglas? :dubious: :o
Depends. Elly May wouldn’t be bringing along any of her critters, would she?
Lol… idk, but she always knew how to great people… one finger and a crooked smile.
They’re all out by the ce-ment pond.
Huh? 
Lisa, because she can cook too. Hotcakes!!