That is so stupid. I've heard of occasions of it happening where random strangers have said "oh doesn't [presumed dd but actually child of someone else] look like you?" but this seems like a sustained propaganda campaign, the bloody loons.
It's like they almost think that your ex (arse) is passing her off as your child when actually it's the gf's?! How mad is that.
I'm just trying to think of a nice way to spin it for your poor dd, maybe something like "it's because they love you so much they would like you to be part of their family" or something?