"Fit and finish" as its come to be related to an AR has nothing to do with accuracy, it's just a marketing gimmick that leads some people to believe they are getting a higher quality AR when they are not.
A better trigger may benefit a good shooter, but it won't help a bad shooter.
Ammo will certainly affect accuracy.
NRA Certified Pistol Instructor & RSO, IDPA Sharpshooter, Project Appleseed Rifleman
Member NRA, GOA, SAF, VCDL