My mum doesn't know. She knows I draw ponies but I draw anyway, and I think she just assumes it's a bit like enjoying drawing manga, if you know what I mean. I still have a lot of merchandise from when my sister and I were in to my little pony aged 7 and 4; I got back in to it a few years later, as a pegasister! (But I don't use all the merchandise, actually.)
I don't think my mum would mind my liking mlp (although she might think it a bit odd) but I think she would mind the fact that I was watching mlp on the laptop when I said I was using google translate for a project I was doing for fun. (I did use it for that a bit as well!)
My mum doesn't like technology much, so we don't have TV, and I'm mostly meant to use the laptop and my phone for work and emergencys, which I do, most of the time!
So, no, my mum doesn't know I'm a brony. Maybe I'll tell her at some point, but not right now!
One of my friends knows though, and she's fine with it.