I absolutely love it. Love it.
I used to have a cat named Zinnia. She went nameless for two weeks and finally I sat down with a gardening book I happened to have from the library and decided I'd find a name in that book. Flipped through alphabetically and didn't decide until the letter Z. Zinnia? Yeah, Zinnia will work! And it did, I thought it was a great name. I did call her Zinny sometimes. It's a pretty natural nn.
And yes, that was for a cat but I still think it would be a fantastic name for a human. The letter Z is so spunky. Flower names are so in style right now, and in my humble opinion, Zinnia is FAR more interesting than Rose or Lily or Violet.
It's honestly a name I can't believe more people don't like. I think it's terribly overlooked.