It's not really all that popular. Or at least, it doesn't have any kind of huge community. There's a small-ish online community, but it's mostly just a silent audience. I'd say most people who like it, like it because it's unapologetically cute, with a dash of "what the fuck..?"
For your other points, I guess it's just a matter of personal taste, perhaps?
I love the art style myself. While it's simple, it's a good kind of simple. I wouldn't be surprised if it drew heavy influence from Takashi Murakami's work, which is very colorful and bright, but also psychedelic and at times unnerving.
The plots aren't exactly Dostoevsky, but I think they work very well. Simple I'll admit, but also really fun, and most of the episodes seek to expand upon the given universe, which in a first season is pretty smart. The last few episodes of season 1 are definitely a bit quick in their pacing, I'll concede on that.
I can't totally agree that all the characters are one-note. Some definitely are, but most of them are written to subvert the one-note expectation. All the characters DO have their general personalities, but with some nuance that gives them a little more depth. Plus, most of them experience some manner of development that shapes their character.
But anywho, at the end of it all, if Hanazuki's just not for you, that's fine man. It's not gonna be for everyone just like ponies aren't for everyone.