I think its just one of those things that are not expected to be cool but end up that way.......
The fact that the show is, cute and pretty, but still addictive with such likable characters makes it cool.......It takes elements from what would be a good show : EG, Good voice-acting, Good writing, Good character's (as in likability and design) and mixes it with elements that would not usually be considered "cool" by most people : EG, lovely Ponies, colourful "ponyland" settings, suger, spice and everything nice (that kinda stuff).
And when they are mixed together they can create a really great show and something that is simply addictive, cool and enjoyable for a wide range of audiences, no matter the gender or age.....(lol that being said, why shouldn't anyone like ponies, or colourful stuff? I don't think your choice in a favourite animal or colour setting should determine your "manliness" or "maturity.")
Compare it to a badass demon/man, loving strawberry sundae's, on paper it doesn't seem like a cool thing, but when you think of Dante from Devil May Cry doing it, it just seems to work now doesn't it?