This is definitely an interesting idea...While I agree with lordbababa's comment that this show in all likelihood wasn't pitched to target older males and people experiencing depression, it's definitely caught that audience..Definitely for the better.
As someone mentioned, the show is fairly 'classic' without being cliche. It is just very lighthearted, the producers know what the show is, and it just doesn't try to be something it's not, which only adds to its charm.
I think another thing is that MLP has developed such a huge fanbase, making it a community of bronies and pegasisters, and communities are very beneficial to mental health. In addition, there is interaction with the creation of the show itself, as the show makes it a big point to interact with the members of the fandom community. This again, leads to reducing feelings of 'isolation' from the television, and a lot of industries these days, it seems...
But it definitely would be interesting to see if there has been any scientific research on MLP FIM's effect on mood states....hmmm...