I could write my own lengthy thread about this issue, but to summarize it, Janna and Lulu are really good at combating the current climate of League that has festered it for years now, and this goes back to my long, long, long, long, LONG track record of bashing overspecialized teams. Teams composed mostly or even ENTIRELY of AD melee assassins and ADCs get crushed easily when they meet a hard counter, and when I say hard counter, I mean so much as a single skill could ruin all of their plans forever, and there are dozens of those kinds of skills in the champion roster overall.
In the current climate, people want to get all of the kills and pentakills forever, and no matter the champion, they use only one tactic; bum rush them no matter where they are and kill them in one burst. If it doesn't work, continue doing it, even though each failed attempt makes it difficult every subsequent try.
It's the tactic of a rabid mongrel.
Lulu has the arsenal of slows and crowd control at her disposal combined with all of her buffs and of course has the best synchronization with the Censer (not getting it with her is like not getting a Thornmail for Rammus. Like why would you do that?). She can also highlight targets with Pix (extending the range of her Q, which is a line slow that can fire in complicated directions), polymorph enemies making them temporarily helpless, and ult allies with a field slow, health boost and a crowd knock-up, giving them one hell of a mulligan as they fight back with their second wind or use the temporary chaos as an opportunity to run the hell away.
Janna has her line knock-up, which can temporarily immobilize an entire lane of minions (and later in the game can wipe out an entire line, allowing her to defend a lane by herself), her passive mobility that allows her to zip across the map with blazing speed, her targeted slow, a powerful shield that gives a huge amount of bonus attack damage, and of course the field knockback and crowd heal that can give an entire team a second wind in a fight (PUN INTENDED).
Are they overpowered at the moment? Probably, but it's really difficult to tell when the opposition is playing directly into the strengths of these supports.