Actually, it matters more at level 100 than lower levels. A nature will make one stat 10% higher than normal and one stat 10% lower.
So a Chandelure with an attack-boosting stat will have 10% higher attack, which is a bit useless. You would be better having a 10% boost in special attack or speed. And since you won't use the attack stat, you can get a nature that reduces that instead. In short: Modest or Timid.