Miss Fortune is a Viable ADC that's been reworked a couple times but never to a point where she IS LCS material and if she reached that point they nerfed her the very next week.
1. Miss Fortune is a lethality based champion, basically the main reason shes viable is also the main reason she will never be LCS material. Miss Fortune is an Attack Damage Caster. A spell based champion with no real other reliable features. She cant go crit because she has no escape, no snare, and since she is spell based her skill damage would be less than impressive. This requires Miss Fortune to go either AP OR Lethality, well AP Miss Fortune got nerfed into the ground during season 7 when she was rising in the support roll meta when zyra was a beast effectivly removing her as an AP option. Leaving her with lethality because of the effect it has with her high damage skills, the main problem is lethality got reworked to benefit assassins not ADCS.
2. Miss fortune is weak against almost all of the classes that exist in league of legends including over 70% of its champions. She cant hold her own against a tank shes squishy dosnt have armour and due to her low base damage after the first auto attack she does no damage to the tank in question outside of her skills . MF cant hold her own against assassins. with no escape and no reliable cc miss fortune is basically paper vs an assassin she will lose every time. MF cant hold her own vs a mage, almost every mage in the game has some sort of reliable cc in the form of a knock up stun or displacment that causes the enemy to not take action for a moment adding on top of that the fact taht magic scales higher than ad does. all of her skills will lose to a mages skills in fights. Bruisers dont even get me started.
3. Miss fortune has to many weaknesses in league of legends. Miss fortune cant hold her own against the above mentioned groups and according to riot, they will always keep assassins and mages viable during these constant changing metas to keep things intresting, concluding that as long as miss fortune continues to have no escape, no reliable cc, no reliable mobility, she will be dead to pro players when the time comes to take action.
FOR THE RECORD I AM STATING MISS FORTUNE IS NOT LCS MATERIAL I AM NOT SAYING SHE IS NOT VIABLE IN RANKED IN ANY SHAPE OR FORM BECAUSE SHE IS, SHE JUST WILL NEVER BE PRO PLAY MATERIAL, ADC PROS LIKE SNAEAKY HIMSELF HAVE STATED ON STREAM, SHE HAS TO MANY WEAKNESSES, SOME OF WHICH I HAVE MENTIONED.