I look childish as hell.(especially being as small as I am.) And I'm sure visiting Aikidoka might not take a pig-tailed girl as serious... but whatever, it is practical.
I'm with you on the pig-tails - I will often do the same whenever the mood strikes me, for the very same reasons of practicality; plus it looks cute and I think I can still "get away with" the look. Though that will definitely change in the years to come...
But as for folks not taking us seriously (I'm 5' 1" and about 100lbs myself) while wearing pig-tails, I have fun giving them a reason to do so. Maybe I just find the ironic juxtaposition of it amusing - in demo videos it always looks like people are getting beat up by a little girl.