Sheesh...I never thought about that, actually I think she might be or might not because technically, her mother is from that place, but her father was not, so either she inherited all her mother genes or there are mixed genes...
Considering how she is shown to age i don't think she is. I think before Elsa many, many years ago, there was another 5th spirit but they passed away (thus some believe or think it's a legend), leaving the spirits sorta "lost" but still okay. It wasn't until King Runeard fully destroyed the peace everything went haywire. Iduna's selfless act of saving Agnarr gifted them both with the new 5th spirit as their daughter. I haven't watched the film since November so i don't know if im completely correct but eh. This is all my theory.