I looked into Bronies for a time, so I'll shed some light on what's going on.
The series briefly involved Lauren Faust, who has worked on shows such as The Maxx, Powerpuff Girls, Codename Kids Next Door, Iron Giant, Foster's Home for Imaginary Friends, and other projects. Now despite some of those titles, if you look into her work she's actually surprisingly good at making the most of some hard to swallow mediums. She was the only thing making My Little Pony even remotely tollerable, which becomes more than obvious when she leaves the show after a season or two. A common theme in her shows is repressed psychological damage in the characters that each have their breaking point spotlit for the audience to enjoy at least once per one. She's also quite good with comedic timing and some entertaining appropriations of comedy tropes.
The show's popularity sprung pretty hard with the help of 4chan. Whether ironically or literally enjoyed, they could not shut up about it. They ran away with it to the point of snagging background ponies the computer auto-generated and gave them fan-names, like Derpy Hooves who was known for a computer draw error that caused crossed eyes, and Lyra who was known for sitting upright in a scene with no lines, and occasionally a referential take such as "Dr Hooves" being a Dr Who parody. There's fanfic writings about background ponies all over the place from the fandom being so over the top that it inspired others to follow suit. Once the fandom had more college kids than children watching it, they began to cater to those fans instead, something that proved surprisingly easy to do since some of the writers are fairly internet savvy.
Inevitably the fandom began making names for themselves (Bronies, Pegasisters, etc), and from there we began to see more genre splicing of subcultures. This means it began to blend into crowds such as Furries in a more than obvious way. It was an easy to appropriate piece of pop culture that could feasibly blend with almost anything, so naturally certain types began to run their own direction with it like what's become of some of the darker corners of Sonic the Hedgehog fandom. Most fandom when it hits critical mass tends to go in some bizarre directions, My Little Pony simply sticks out because it's a girly children's show doing it with bright and gaudy color choices. Most of the fandom has passed, but the diehard crazies can't seem to let it go that their show's essentially been dead since a few episodes into season two at most.
My personal opinion on the show? The first season and a little of season two is somewhat tolerable if you ignore the baby-character arcs and zone out through the really hard to stomach songs. With company it's a little more tolerable as long as they don't fucking sing along with the damn songs, as it makes it easier to wait for the parts where the cast go insane. The little things like referring to "everybody" as "everypony", a town being called "Equestria", and the Mayor being named "Mayor Mare" start off a bit painful, but after the sheer oversaturation of pony puns it becomes B Movie level comedy where it borderlines on the absurd.
If you want an idea from a fairly well informed outsider, I reccomend watching the episode of Bob's Burgers titled The Equestrinauts. It sums up some elements of the fandom pretty well with parody by showing it through the perspective of a non-fan having to be a mole within an offshoot of Brony subculture.