What's offensive of acknowledging the existence of a date and a person?
birthdays are known simply as happy days of joy, to celebrate another year in the life of someone, whether they are dead or alive. notice how all around the world, people celebrate the birthdays of people such as martin luther king and gandhi, because of what they stood for and what they gave to society.
and, well, you know what Riddler stood for.
i get that what you did was a joke, and some people could take it, but some other people, whether they have relatives that were in the holocaust or (i highly doubt this) were in the holocaust themselves, might take offense.