"Ragg Mopp"? Never heard of it.

"Itsy-Bitsy, Teenie-Weenie, Yellow Polka-Dot Bikini" I doubt if many people outside the generation that grew up with it will be likely to ever sing it. I should probably have included that in my rule of thumb. Out of the thousands of Victorian parlour songs and music hall songs, or only a relative handful have made it through in that way. And the same goes for other types of soings that were populat in their day.

If it does pass that test, I'd class it as a folk song - well, I did say that there are some folk songs that aren't much good.