Are offensive decals allowed on POVs on military installations?

Within reason and certain guidelines, you have the right to plaster whatever you want on your personally owned vehicle. Your Commander also has the right to tell you to remove them or park said vehicle outside the installation and walk to your appointed place of duty.

A person enters the military, their rights change. Certain rights are infringed on and the courts have upheld this.

What is Free Speech protected for a civilian may likely not be for a service member. Never assume otherwise. A military member is bound by the UCMJ, DOD and Military regulations of which there are a plethora.

An enlisted member is a public employee, not a private citizen. Choose carefully which hill you choose to try to take, it may be the one you die on.

Are they allowed on installations? Yes. Are they allowed on the vehicle of a servicemember? That is up to the Commander.

Once upon a time I had to duct tape over my bumper sticker that said "Honk if you haven't slept with Clinton." It was determined to be derogatory to the President (rightly so) and thus a violation ofUCMJ. I was allowed to keep my "I'm not wearing underwear" bumper sticker, despite it's poor taste, because it didn't technically violate any rules (other than the rules of decorum). These days I wouldn't have either, but back then I was a young, dumb Private.