When a move scores a critical hit, damage is calculated based on either the modified or unmodified attack and defense stats, whichever is more advantageous to the attacker. For example, an attacker who has used Swords Dance and then makes a critical hit will get the benefit of the increased Attack, while one affected by Charm will use its normal Attack score. Another example would be if a team had Light Screen or Reflect on it and the attacker scored a critical on the team with the certain wall; the defense added by the wall would then be ignored, and the attacker would do double damage. However, it does not ignore modifiers caused by items or abilities such as Eviolite, Defeatist, or Slow Start.
Yes, critical hits do bypass Light Screen and Reflect but it does not ignore items or abilities.