I've noticed that a lot of my daughters friends put on American accents when they're playing; we're in the UK, so presumably they just watch a lot of american tv. I don't think it's particularly alarming though, I wouldn't think to warn someone to 'be careful' of letting their kids watch My Little Pony because they might imitate the accents
That's funny, I'm in the US and my daughter puts on a British accent when she's playing.
I guess for either one it's something that sounds exotic and fun.