I got a tattoo a week after diagnosis but I would recommend talking to your dsn/gp before hand just in case they have any objections at this stage. I can't see it being a problem but they can give some hood advice on aftercare etc. Always take a sugary snack with you to the tattoo parlour, I find my BG drops every time I have one done my weapon of choice is a Mars bar

also its highly unlikely to happen but as with all activities that pierce the skin please take extra care and watch out for any sign of infection as the sooner it is treated the better the outcome will be. I'm sure others will be able to shed more light on the subject but thought I'd offer you my snippet of advice