Her ult does leave a lot of be desired, I don't think doing more damage at closer range is the way to go. Ask anyone that plays as MF, the worst thing you can do if you are even going to consider using your ultimate is to use it when you're close to any champion.
I wouldn't say she's the worst ADC right now, though she does leave a lot to be desired, in my opinion, she has two things going for her:
She is a pretty strong lane bully. Once you master the Q bouncing, you can destroy your lane opponents simply by the crit bounces.
She's pretty solid at roaming, considering she gets a mobility boots passive when out of combat.
But the one thing that I just don't understand for the love of god is why the frick does this passive get deactivated by minion hits?