Yes, it's the only knock-back in the game that doesn't do damage. Lulu and Diana are the only other two with displacements that lack damage, and between the 3 of these we have one each of knock-up, knock-back, and pull that do not have damage. Seems like a pretty fair distribution to me.
I can't think of a reason Janna's ult would need damage, either from a thematic or gameplay perspective. In game design, wind-based abilities are usually implemented as either large-scale bursts of force, or more focused cutting attacks. In League we have Janna covering one of these, and Yasuo (and Riven's Wind Slash) representing the other side.
As far as gameplay is concerned, are you thinking that Janna's kit doesn't have enough to it? She's a highly defensive support (that can also poke reasonably with Zephyr and Whirlwind if you feel like it) that focuses largely on either mitigating damage as it comes in or inhibiting the enemy's ability to dish it out. She has 2 displacements with good range that are both fairly easy to use, a single-target slow, and one of the strongest shields in the game that aids with both offense and defense.
Just those seem like enough to me, but she also has a good-sized heal thrown in too. There aren't very many champions in this game with both a heal and a shield that can be used on allies. I think as of right now it's just Janna, Kayle (if you count her ult, which you should =P), Sona, Lulu, and (in just a little while) Taric. And Kayle and Sona both have a large portion of their power budget focused elsewhere in their kit, so at least one each of their heals/shields is lackluster. Janna gives up nearly all damage in order to get the best set of defensive abilities in the game, with a grand total of 4 direct life-saving mechanics in one kit.
TL;DR Don't dive Janna =P