I'm a diamond 1 Janna main, and I have over 600 games on Janna. I believe she does need nerfs. It's just the /way/ they're nerfing her that doesn't make sense. Zephyr turned into a weird mini game - Howling Gale (which is already on a long CD) getting an even longer CD, More damage being taken off her kit (not her problem), mana restore added to her kit (also not a problem for Janna).
The only nerfs that really make sense right now on the PBE are her shield CD nerfs, which are deserved. I'd like to see them nerf that and leave Howling Gale and Zyphyr alone, and take a look at her passive. Right now her passive is adding a ton of invisible power when stacked with Coin, which is probably where they should be looking instead. That, or her shield duration.
Riot has been nerfing Janna's damage for years and it's never fixed her issues. It just makes her feel increasingly flimsy, and the Zyphyr nerfs will definitely make her feel a lot worse to play with.