I fucking hate Hotaru. In a cast full of colorful characters with actual personalities, you have this cunt whose entire character boils down to "lol I'm physically attractive." That's it. It gives the otherwise classy Non Non Biyori an element of fanservice that doesn't belong.
Hotaru's also kind of an ugly person deep down because she derives a sense of superiority from the fact that she's prettier than any of her friends. In the city Hotaru was much more average, and she didn't like that; she prefers being around a bunch of homely country hicks. The beach scene was basically just her bragging (under a guise of innocence) that she's only 11 yet has larger breasts than any of them. As time goes along and her boobs get bigger and bigger each year, you can bet this kind of behavior will become increasingly more common. 16 year old Hotaru will be like "Natsumi, will you come with me to the city? They don't sell bras my size anywhere around here. Sigh. You're lucky to have such normal sized breasts, I'm so jealous."