Hey There. I am a Miss Fortune main player.
Its true she can blow more than half your hp with one q or AA, but that is only with 2 or 3 items and dont forget ITS ONLY THE FIRST AA. Once this strong shot is used, her damage is really low compared to ANY OTHER ADC. And anyway you can dodge the q, just dont stay behind minions with low hp. It is not hard to land but its even easier to dodge.
Besides that, shes can only use that strategy on squishy targets, because as she needs to build lethality she doesnt deal serious dmg to tanks or offtanks.
Dont forget shes the adc with THE LOWEST initial ad value. With domination and sorcery she barely reaches 70 ad at lvl 1, when most adcs have at least 76, some of them even 80. If you play her with the regular adc runes, she has 64 ad with doran blade.
Now, w is not mobility. It is a very tiny movement speed buff that can save you sometimes, but it is useless vs any chamo with a gap close, amd almost half of the champs in the game have a gap close.
Its funny to destroy a xerath or a twitch in 2 Aa, but if that Twitch survives that first shot, Mf will lose the trade.
I admit shes strong in lane phase too. But thats is because you are afraid (and she knows it), and if you play passive then she will have a lot of chances of landing her q (wich only deals big dmg if she has dark harvester on). You need to pick a badass adc like Xayah, Lucian or Draven. I am a Mf main and I NEVER let Draven open. It is just an impossible match if he knows his stuff. It works in the other direction too, so dont pick Vayne or Twitch if you dont wanna be destroyed.
Now, the ultimate. It is not a noob champ skill, it needs synergy with teammates and it is risky. It leaves a no mobile adc stopped in one place during almost 4 secs. Any good zed, kayn, riven, jax etc will jump on Mf face and instakill her. Just ward good if u are doing baron and stop crying.
And you can easily interrumpt it or protect yourself from it. Zhonya, barrier, Braum, the fck....ing wall of Yasuo... Nami's ultimate etc. If you kill 3 people with it is because you had a good positioning and a good teamwork, not because the skill is op.
So, if you usually play Vayne, Twitch or Ashe, dont complain about your counters. Jhin or Draven will blow your hp in 1 sec as easily as Mf, and they are not so killable. If you know she counters you, then ban her.