I don't see the problem with her getting a tattoo. And I don't say this as some big AJ mark but rather as someone just using logic. It's not like she can dream of being WWE Champion. The Women's Championship (which the Diva's Championship now stands in the place of) is the one and only championship that women like herself can win in the WWE. And she's dreamed of being the holder of this one and only WWE female wrestling title for the past 14 years. So she got a small marking/tattoo to celebrate it. Big deal.
If she ever planned on tattooing herself with a date for every single time she won the belt (say she won it like four or five times during her overall tenure in WWE), then that'd be a different story. Just getting one small tat on the back of her neck that isn't even visible when her hair is down to celebrate reaching her lifelong dream isn't an issue, or at least shouldn't be.