This is the first time we see Evey's tattoo. Although its presence had been hinted at in Chapter 4 of Book 1. The tattoo is actually the Mallet's anchor point. The tech that runs the Mallet has physically bonded with them. So Evey's not only the wielder of the Mallet, it's literally a part of them. Danae used to have a very similar tattoo when she was StarHammer.
Evey was really surprised the first time they caught notice of it in the mirror. And although they're not really one for tattoos, they think this one is pretty neat. Izzy had a lot of questions that Evey hasn't and cannot answer.