Ah! I found out about this kind of thing several months ago, from a video I saw posted on the internet that was from an English-language series about real-life pop culture trends in Japan. I had forgotten what these types of cars are called, but after a couple internet searches, I was reminded that they're given the Japanese name
itasha.
And yes, this trend of putting decals on your car to show off something you're very much into is a popular thing in Japan - both back when I saw that video and even more-so now. The latter was evidenced by the article I saw (in this internet search today) about a popular gathering spot for showing off your
itasha. See http://en.rocketnews24.com/2014/05/02/every-weekend-is-an-itasha-car-show-at-akihabaras-udx-parking-garage/ . The article doesn't refer to (or show) any Sailor Moon-themed
itasha, though it does show a CCS-themed one (however, the way this article's author refers to CCS isn't exactly flattering
).
BTW, even though I've long hated driving, I wouldn't mind tooling around town in a Sailor Moon-themed
itasha (slowly, so that I make sure to not crash it
).