I think these are actually called Bindis
And while I don't see anything offensive in it as a costume (or as a style statement honestly),I am not Hindu nor am I of Indian decent, and thus can't really say. However I have seen some people who take extreme offence at non-Hindus wearing Bindis. Here's a good source
Just be careful. You don't want people like
going after you. (Though I do suggest reading her links, if you want to know more)
As I said. I can't say if you should or shouldn't wear one. I just wanted to make sure you made an informed decision. (:
Edit: Ooops took to long to write my response. ^^'
Oh well. That's my 2 cents on it.
(also I agree that the third Character just looks like a tattoo, so that should be cool. (: )