Fortune is somewhere in between. She averages in everything while specializing in basically nothing. She doesn't have any fast mobility, but her strut gives her a sizable movement speed bonus, which is very handy for escaping. If you can aim your shot right with Q, you can kill a minion with it and ricochet the bullet right into an enemy champion's face, which is a guaranteed crit. Her W is a modest attack speed boost while boosting her damage against opponents she tags multiple times. Her E provides a little bit of slow, which is handy for suppressing enemies long enough for you to close the gap or get away. Her R is one of the very few ults for ADCs that can hit multiple targets multiple times while also increasing the damage against targets too stupid to get the hell out of the way.
In short, Miss Fortune is fire control. She excels in a defensive style where she can keep the enemy at a distance and take pot shots at them.