Excluding assassins (and a well played Sion/Olaf), she pretty much invalidates melees when played even semi competently. She's nearly auto-win on ARAM and 3v3, since there's barely any room to dodge her spells.
Her top tier spot on 3v3 is interesting, because the map is generally overrun by bruisers and melee carries, both of whom should have an easy time dealing with her, according to the boards.
She's at the top of every map's tier list for a reason. I mean, the numbers can't lie. I'm not specifically qq'ing, but I think Lux is a lot stronger than people give credit for.