You can get DF'd for anything 3 elders agree on. That said, I've never heard of anyone getting the boot for a tattoo, but usually it's one of those things that people do while they're inactive and if they come back the elders don't push the matter.
If someone were to be going to meetings regularly and one day showed up with a very obvious tattoo, I suspect they'd get a good talking to, but it depends on the congregation and their response whether it would become a judicial issue.
The majority, by far, of JWs that I've know to have tattoos were converts and had them before they got ensnared by the cult. In that case, no one cares.