As a guy who watches KND as a guilty pleasure, I suppose that if you are 'mature', you decide for yourself what is 'appropriate' to watch. Really, this whole issue with MLP (not that I watch it or advocate watching it) can be broken down into the following:
1) Kids spend their entire lives waiting to grow up and enjoy all the things adults get to enjoy. It's only natural for people not to understand why someone would stick with kid's stuff when they've got all the adult stuff at their disposal, and people don't like things they don't understand.
2) At no point in their lives were these 'bronies' ever the target audience. In fact, the target audience has a set of values and interests almost directly opposite to the age category the 'bronies' are in. And what's worse, they never acknowledge this or that it seems odd they like it.
3) The show has got cute talking ponies.....and nothing else worth noting. Yes, there's this apparent 'continuity' between the episodes concerning the ponies and the world they live in, but everyone just watches it for the cute ponies. And that's just childish and feminine.
4) All stories, whether they be for little girls or old men, revolve around conflict. Conflict is what makes the story interesting. What conflict there is in MLP is dry and meaningless. Ergo, they have no good stories.
5) I have seen a great number of the 'bronies' act like immature asshats. I'd say its safe to assume that most of them are immature, asshats or a combination of the two.
So, yes, at the end of the day, it is a show for girly manchildren. If you want to go out and watch it, well I guess no one can stop you from being a girly manchild.
Note that while 1) could be applied to a lot of different shows (Mighty Morhpin' Power Rangers, Loony Toons), people usually disregard it when the show has redeeming values (parental bonus/jokes that went over your head when you were a kid, classics/nostalgia, B-movie type goodness, really good art or writing, etc), which MLP does not.