It's very rude and not appropriate to dictate to others which threads they should not partake in, and equally rude to tell others that you don't value their input. It would have been fine to say, "I'm not talking about the names themselves, I mean the alliteration", but it's not fine to put it the way that you did in your first two sentences. And few are going to be completely unbiased about the name
Jordan anyway. It's not like we can't dislike the name
Jordan and have a separate opinion about the alliteration. And if someone here and there does throw in an opinion of
Jordan, either in addition to or instead of an opinion on the alliteration, what's the harm done? It's not like you can't ignore it.
I don't like
Jordan for a girl, but I don't think the alliteration between
Josephine and
Jordan is a problem. More of a problem is the stylistic difference between them. Parents who choose
Josephine are not that likely to also choose
Jordan, and vice versa.