The unique thing about ADCs is that they are all so similar that they're all viable. Urgot, being the most dissimilar, obviously doesn't count.
Jinx is popular because of her incredibly strong mid game, the same reason MF is popular. . She has snipe potential, hard cc, wave clear that scales with AD, and splitpushing potential galore. It's a quite effective package. Graves is popular for the same reason, his dueling potential from his passive is unmatched, excellent wave clear, an escape, and a slow. His AD scaling are also very good. Personally I find his long attack animation a bit of a hindrance, which is why I prefer Jinx.
Basically, it's not that MF or Varus isn't good. They are designed to thrive in this meta and can. It's that she's outclassed by champions that do what she does better, as well as doing other things too. I mained MF before Jinx. It took a bit of an adjustment period, but MY GOD is the difference clear as day now.
Also Corki. Corki is very strong right now.