Lulu Support is my favorite, but I am still learning so take anything I say with a grain of salt!
I try to save my E for shielding. Generally I will only throw it on an opponent if I am trying to increase the range of my Q by shooting from Pix into a bigger group of the enemy. Tricky, but fun when it works! Otherwise I find the benefit of the Shield on an ally (or myself) is a better use of the mana. Are you able to keep your mana regen up enough to keep harassing with E? If so, there shouldn't be a problem, it just doesn't work so well for me, could be my relative lack of experience.
For poke/harass I stick to my Q, I love that it has a slow effect, plus range can be changed/increased when you have Pix on an ally or opponent so it can tag them when they don't expect it. Auto attack is also a great tool, since it has some range on Lulu and can save you some mana in the early game.
If I feel like someone is getting too aggressive on my carry I love to throw a W polymorph on them - kind of a way to say "You want to play with us... try playing as a Kitty!" (Squirrel or Cupcake, if you're not using the Wicked skin, but I always play Wicked Lulu for now.) I do need a little practice using Whimsy as a get away tool, for me and my carry - I just love to use it for crowd control SO MUCH! It's fabulous if you can follow it up with a Glitterlance to slow them, too. In that case they are set up really nicely for some serious damage.
As you can tell, I think she's a totally viable support in ranked games, especially if paired with the right carry.