It's probably not. Federal employees - these guys are apparently Dept of Homeland Security (although who are "DHS Police"? What job is that?) fall under a bunch of statutes and regs collectively known as the Standards of Ethical Conduct, but those mostly focus on endorsing (or appearing to endorse) in an official capacity, or use of position/title to endorse (or appear to endorse) personal activities.
Even if DHS Leadership completely loses its shit after seeing this photo, it's a loooong stretch to claim either of those provisions here. DHS also has a supplemental reg of their own, which presumably includes stuff regarding enforcement officers in particular, but also keep in mind that these guys have a union, so they can't just fire them for causing mild embarrassment to the agency. People take selfies with celebrities while in uniform all the time, and Mia Khalifa's an actress, not a criminal. "Stern admonishment" is likely the worst thing that could ever happen to them.
State and local laws are usually similar, but they might be enforced more strictly. Still, I strongly suspect there's more to Officer Miller's story, because that's not typically a fireable offense. For example: Was he responding to a call at the bar, then ended up just screwing around and taking a photo with the bikini girls? Worse, did he let someone off with a warning for something the usually leads to an arrest? Or did the photo, say, also involve him chugging a beer in uniform while Taylor Swifting one of the Miller Lite Girls? Or did people there witness other "poor decisions" that weren't captured in the photo?