I would've just dismissed it as "characteristically random" even without this clarification. The Japanese are known for using western religious imagery/references in the most innocuous, secular ways--just look at how they adopted the holiday of Christmas into their culture.
If anything, I find this situation a bit less interesting now, because it went from "WTF random" to just another humorous shirt.
Of course, she still wins because it's likely she understands the joke too.