Not sure about the tattoos, but I wear a pony tail because I have long hair. Some people seem to like the tattoos though, my niece has 3 tattoos but she is not a biker. :s
I have been on motorcycles since 1975 and have also served in the British army for 25 years and did both without tattoos and no pony tail although it has crossed my mind once or twice to grow hair and have a pony tail wife said no although i get to keep my beard as long as it does not get out of hand
Culture & tradition...same reason us retired military troops still cut hair short, are punctual and listen to orders. You'll find different opinions from many & it's not really because they're bikers...it's because they like the style/icons & the fact that they can...JMO.
My hair goes down to my elbows. If I don't braid it into a pony tail before I ride, then it gets all tangled in the wind. If I don't braid it all the way to the end, in a bad wind hair will fly up and get in my eyes.
I have a tattoo on my arm that is hidden by any T-shirt. Years ago my sweetheart & I got tattoos together on Valentine's day. She got a heart on her butt, & I got a pirate. I have not seen her in a long time, but I still like the tattoo.