Is Sneak Attack damage maximized on critical hits? Short answer: yes.
For the long answer, first read what the Players Handbook says about critical hits on pg. 278:
Maximum Damage: Rather than roll damage, determine the maximum damage you can roll with your attack. This is your critical damage. (Attacks that don’t deal damage still don’t deal damage on a critical hit.)
Extra Damage: Magic weapons and implements, as well as high crit weapons, can increase the damage you deal when you score a critical hit. If this extra damage is a die roll, it’s not automatically maximum damage; you add the result of the roll.
Furthermore, according to this FAQ:
10. Which dice do I maximize when scoring a critical hit?
Only the dice you would normally roll to calculate damage are maximized. If another bonus (like from a weapon or feat) causes you to roll extra damage dice when scoring a critical hit, those dice are rolled as normal.
Given all this, I think that Sneak Attack damage is maximized on critical hits. The main point being that the definition of “Extra Damage” in this context refers to damage that occurs as a result of the critical; Sneak Attack occurs whether the attack is critical or not, therefore it is maximized.
The rules are clear, but I decided to write up this explicit answer given the confusion I’ve seen in various forums about this question.