If I was the mother with the baby, I would rather you asked the question out of interest than just stared.

I think you did the right thing.

my little girl has one in the middle of her forehead too, and i dont mind people asking, especially whe they have children with them, becaus ethe more people that know the better. As soon as I explain that it will go away with age, kids are over it and not interested anymore. Id rather people asked instead of staring at it, because even as a one year old, my wee one knows when people are being a bit odd lol

I think you did great.  The mother probably appreciated you asking about it, rather than just staring. 

I think it's fine--if you had said, "Ew, what is that thing?" that would have been rude.

Not at all; absolutely appropriate! Sounds like you handled it well.

If it were me I'd be glad to've had the opportunity to talk about it.

Personally I don't think it was inappropriate. You were just curious and it seems as though the mother was fine with informing you.