I don't really mean giving them
exactly the same, but IMO there should be something, especially for higher CR foes.
For example. In our house-rules, a nat 20 with a bludgeoning weapon can stun the target if they fail a STR save. Well, we were were facing Hill Giants, not only did the DM have them deal maximum damage (instead of rolling) of 29 points, but we had to make a DC 16 STR save or be stunned until the end of our next turn. All I will say is "OUCH!". Most of are party was
not proficient in STR saves, like my PC with--oh--a STR 8! At the time that was about half her HP IIRC as well.
So, while I am all for making critical hits "cooler" and "more unique", sometimes those things (or similar ones) will hit the PCs... and then you have to ask, "Is this really more fun for the players?" Some will say "Yes, I love it!" while others will not.
All that being said,
class features that improve critical hits are different from general rules (such as our "critical = possible stun"). I agree, class features don't need to be represented in NPCs any more than every NPC feature has to be represented in a class somewhere.