According to the Pew Research Center, about 14 percent of all adults, and 36 percent of people between the ages of 25 and 29, are adorned with at least one tattoo. But, like Crocs, just because tattoos are popular outside the booth doesn't mean they're always appropriate inside it.
Before telling your staff that tattoos are taboo, your company should first establish a formal and written "No Visible Tattoo" policy. Such policies exist in literally hundreds, if not thousands, of businesses and can help clarify any issues that arise over body markings. The law still tends to support company dress code/appearance policies in general, while allowing employers to retain some flexibility. In general, that means these laws permit employers to create policies requiring workers to present themselves to the public in a way that is consistent with the employer's desired image. But, given how widespread tattoos are, such policies in your case don't have to exclude any worker from having or getting a tattoo, just flashing it in the booth.
The key to handling this successfully is a written policy that employees must read and sign off on. That way, when the employees are working your booth and complying with your policy, attendees will focus on their skills, not their skull-and-crossbones tats.