As I was raised in a Hindu family, I guess I can offer my two cents on the topic. Due to the costume itself, a monk it shouldn't be offensive at all. It makes sense in terms of the costume. Those blogs that were linked(I did a quick skim) seem to focus on girls who wear it with Western clothes and culture.
Since it is being done with a costume where a bindi would be approiate I don't see anyone taking offense. If they do, you could easily take it off if you feel uncomfortable.