I play with MLP's on a daily basis, because my 3yr old demands it.
That makes me question the intelligence and relative level of... mental growth (there's a better word for this somewhere) of grown up MLP fans. However, going on a crusade of hate against them for liking something I can't fathom to be likeable is not really my style.
Have you seen the show or just played with the toys?
Most fans are pretty disappointed with the toys, as they're pretty much the epitome of "pink sparkly crap." Everyone shakes their head at Applejack's pink apple shaped truck, for example.
As for "mental growth", well, here's the famous CS Lewis quote:
Critics who treat adult as a term of approval, instead of as a merely descriptive term, cannot be adult themselves. To be concerned about being grown up, to admire the grown up because it is grown up, to blush at the suspicion of being childish; these things are the marks of childhood and adolescence. And in childhood and adolescence they are, in moderation, healthy symptoms. Young things ought to want to grow. But to carry on into middle life or even into early manhood this concern about being adult is a mark of really arrested development. When I was ten, I read fairy tales in secret and would have been ashamed if I had been found doing so. Now that I am fifty I read them openly. When I became a man I put away childish things, including the fear of childishness and the desire to be very grown up.
If anyone is lacking maturity, it would be the ones rejecting it due to being "kiddie stuff."
Re: Gore fics. I don't see how it's any different than say, slasher films. Some people just like gore. And yeah, the shock value is (suppoed to be) somewhat increased with cute ponies instead of boring old humans.