Sì: The phrasing for critical hits in 5e is the general attack. This counts for anything with an attack roll, a monster attack, an adventurer attack, a generic NPC attack and a wizard (of any stripe's) spell attack roll. Here's where it talks about a natural 20:
If the d20 roll for an attack is a 20, the attack hits regardless of any modifiers or the target’s AC. In addition, the attack is a critical hit, as explained later in this chapter. (Basic D&D p73)
ATTENZIONE anche al concetto di critico:
When you score a critical hit, you get to roll extra dice for the attack’s damage against the target. Roll all of the attack’s damage dice twice and add them together. Then add any relevant modifiers as normal. To speed up play, you can roll all the damage dice at once. (Basic D&D p75)
The dice rolled are doubled (static mods are only counted once).
Just to be clear they double dice rolls not the result of the die roll. So 1d6+3 damage becomes 2d6+3,, not the result of 1d6+3 damage times 2.