A lot of this is skill base. If you compare the ability to climb a tree between a monkey and a fish it's really unfair. This is very much like orange and apple in my opinion. For example, if Ahri's playing against LeBlanc and LeBlanc can nuke Ahri down does that make Ahri out gunned completely? I mean they're both mobile, they're both mages. But ultimately it comes down to scaling factor, utility factor, cool down factor, and overall it's just a difference in kit. If you compare Lux to another utility mage such as Karma, Morgana, or Lulu, Lux would stand among the higher damage factor in my opinion (haven't test this). Also, back to the Ahri and LeBlanc match up, I've seen Ahri out perform LeBlanc and vice versa. Player skills play a part too. Can you honestly say you guys are completely even in skill? If you're punching raw numbers alone, how do you incorporate Lux's passive and Lux's skill into the equation? I'm sure mana regen is also a factor for Xerath too. Whereas if you compare Xerath to a DPS mage like Vel'koz, they're pretty close to each other in terms of damage output. I daresay Vel'koz wins by a small bit. But does that make Vel'koz better? No. The range factor too.
TL;DR, I feel that Lux isn't out gunned, but in this comparison of DPS, sure she is, but it does not mean that Xerath just overall better than her.