You don't need to poke with your Q. You're going to run out of mana very quickly if you do that. All you need to do is poke with autos in between last hitting. If you notice that their heal is up, poke with your W up to cut healing in half. This forces Soraka to either wait for the debuff to disappear (which the ad carry will take a chunk of damage and lack the armor buff) or heal only 50% of the amount.

Also, MF and Caitlyn are two different AD carries with different uses. Caitlyn is strong early game, weak mid game, super strong late late game. MF is strong early game, strong mid game, weak late game.

I love Miss Fortune, really love her, i got 2 skins for her and i play her all the time as my main ad ranged. But to be honest, she could really use a buff in her w (the healing debuff last too little). She is not played often simply because there are better ad rangeds. She can kick the ass of many of the "top list" ad rangeds, but you need to have a lot of practice to be EXCELENT early game and be able to avoid deaths and get fed. In ranked she's very hard to play, but still viable.
She is in the line of "ok" and "underpower" because of:
1) a lot of LOW ap ratios (making ap mf or hybrid mf not too viable)
2) low duration of her only 2 debuffs (w and e), not to mention that the as buff on w is the lowest of all (trist and graves have higher as buffs)
3) she's so much better just using normal hit than spamming skills (she lacks of mana and her skills have ****ty ratios)
4) she needs a good team or being really good positioning for not screwing ulti up
5) if you catch her she has almost 0 escape ability
6) Often the enemies got away safely because you lack of cc

What i would ask to riot is a slight buff on the duration of the cc's she have, or a buff on as steroid, just that, i know how to lane my ulti and where to use it so that's all i need.

I only pick MF against really sustain heavy team comps. I generally pick her if they other team has ww, Soraka, AD Sion, or what have you.

Her passive allows her to return to base early without losing much xp/gold which is good for early game. I see her as pretty much just a counter pick champ though. I would never pick her over Graves/Kog'Maw/Sivir if the other team isn't quite heal heavy.

I play MF a ton, and really for one main reason. Yeah, she has a nice heal debuff. But the reason I like her is just because of the team fight control she can bring. She literally feels built (to me) to just mess with the other team's positioning. Her Q can hit the carry, even if they're in the back. Her E can just lock up the whole other team. Her ultimate just melts people if they try to fight in it. If you E-R in a jungle fight, the other team HAS to disengage or run to another spot. I just can't think of another carry that has that much control, minus Graves, who I dislike just because of preference.

Worst AD carry in the game IMO.
Her abilities aren't that great and her her ult is stopped by all CC except a slow and is very easily dodged.

Mf ftw