Stormtroopers are known to be bad shots, for the most part. The notoriously "terrible" Imperial soldiers have been called precise marksmen by Obi-Wan and managed to take out the majority of the Rogue One squadron, yet they've never been able to shoot the main characters in the crucial moments (save, of course, for in Rogue One). The Mandalorian has taken this to new extremes, with multiple jokes being levied at Stormtroopers for being unable to shoot a target. Most notably, the opening scene of Episode 8, "Redemption," where two Scout Troopers seem incapable of shooting an object.
But are Stormtroopers bad shots, or is there something more? When you break down the variety of Stormtroopers out there, we come to realize that not all Stormtroopers are created equally...and not all Imperial soldiers, it seems, have the best equipment.
WHAT IS A STORMTROOPER?
When discussing this issue, it's important to break down what is and isn't a Stormtrooper. The Clone Troopers who fought during the Clone Wars are not Stormtroopers. The Clones were highly trained and skilled soldiers who managed to execute countless Jedi in surprise sneak attacks. These required true marksmanship and precision. Following the rise of the Empire, some Clone Troopers would go on to become Stormtroopers. The vast majority, however, are not.
Stormtroopers are human soldiers, recruited for battle in the Imperial Army. However, not all Stormtroopers are equal. As of now, the Star Wars universe has introduced various Stormtroopers with vastly different skillsets and talents, as well as different sorts of classes. As is the case in the army, different Stormtroopers are given different forms of training.
With all that being said, Stormtroopers still have a notorious reputation for having a poor shot in the series proper. In The Mandalorian, for example, former Imperial Marksmen Mayfield (Bill Burr) takes issue with being confused for a Stormtrooper. Clearly, the galaxy holds Stormtroopers in the same regard as fans of the franchise.
In the case of the First Order, we have the added "benefit" that each Stormtrooper is kidnapped from birth and put through a rigorous training regime to craft them into perfect soldiers. As we see with Finn, this training results in a highly competent soldier who can adapt to most any new situation. In contrast, Imperial troopers were recruits put through military training, much like how real-world militaries function. It should be noted, however, that the Imperial Military transitioned into being the First Order.
POORLY TRAINED OR POORLY EQUIPPED?
In the clip from the season finale of The Mandalorian, the Troopers in question are of the Scout class. This kind of soldier is trained to fight while on speeder bikes and with sniper rounds, with their standard weapon being the E-11 long-range blaster. As seen in Episode 7, the Scout Troopers easily took down Kuiil while speeding on their bikes. These instances play to their particular strength: drive-by assassinations and sniper fights.
However, they attempt to shoot that can with EC-17 hold-out blasters. These are standard issue weapons for the Scout Troopers, yet neither of them can hit a target. One could make the argument that these soldiers, trained to be snipers and mounted combatants, might be wielding terrible close-ranged pistols. Perhaps the Troopers just aren't trained to handle weapons outside their narrow specialty.
One interpretation that isn't valid, however, is that the EC-17 blasters are bad weapons. The EC-17 blaster is also a standard issue weapon for the Rebels. Both sides use the weapon during the Battle of Endor in Return of the Jedi. Yet the Rebels seem to be perfectly competent shots.
The Scout Troopers in "Redemption" don't appear particularly well-trained. They seem pretty lax and off-guard while in an active war zone, especially compared to their peers. This stands out even more in the prior episode and later on in the finale. In this outing, we see a squad of Death Troopers gun down several Stormtroopers and the Client through a wall, almost slaying the Mandalorian. Multiple Troopers also land shots on the Mandalorian, only for his Beskar armor to protect him. Clearly, some Troopers are better trained than others.
IN THE RIGHT HANDS, THEY ARE PRECISE
The Stormtroopers in the original trilogy are almost across the board easily defeated -- save, of course, for one battle. Throughout the series, the Stormtroopers fail to hit their target, often missing by wide margins. The standard Troopers throughout the films use the E-11 blasters. One could attempt to make the argument that this weapon might not be the best equipment.
However, they're actually an improvement on the DC-15A blasters -- the same weapons the Clone Troopers used to gun down the Jedi. In the right hands, these blasters can eradicate an entire division of Jedi. With improved, updated equipment, the Stormtroopers are inherently worse-off than their predecessors.
This is in part because the Clone Troopers were attempts to clone and recreate the combat potential of Jango Fett, one of the greatest soldiers who ever lived. And in reality, the Clone Troopers were created to kill Jedi. Stormtroopers, though, were never trained to combat threats of that magnitude. The Empire had specially trained Jedi Killers for that, like the Inquisitors. When you compare a standard Stormtrooper to their elite combat unit counterparts, like the Death Troopers, there is no comparison. The Death Troopers are the far more dangerous class.
But when we look at normal Troopers? Most are fairly weak-minded. Jedi Mind Tricks don't work on mentally strong adversaries, yet every Trooper, when dealing with a Jedi Mind Trick, is easily befuddled. Clearly, these guys aren't the brightest bulbs.
THEN THERE'S THE FIRST ORDER
The First Order has perhaps the best system of Storm Troopers -- which is proven by how dangerous Finn is. When we look at the First Order troopers in action, they lack the same weaknesses their counterparts in earlier films do. They have the F-11D blasters -- an even more advanced version of the E-11 blasters. They are trained from birth, making them even more dangerous. We see random troopers trained in multiple forms of combat, with one, FN-2199, tossing down his blaster to fight Finn with a Z6 riot control baton.
The First Order troopers never really seem as incompetent as the Stormtroopers in the original trilogy. C-3PO and R2-D2 wouldn't walk right through a barrage of bullets without coming out unscathed.
But that said, there is also a far stricter code of conduct. Finn is reprimanded for taking his helmet off without permission. The Troopers are trained from birth to be ideal weapons for the First Order. They saw the inherent weakness of the prior Stormtrooper system and went to inhumane lengths to make sure all of their Troopers were as advanced as the Death Troopers and Clone Troopers of old.
Yet at the same time, the First Order troopers are equally vulnerable to suggestion, and, come The Rise of Skywalker, Poe and Finn can charge through a hallway shooting them down left and right without worry. The only conclusion, here, is that the only reason the better-trained, better-equipped Stormtroopers keep missing is that the Force is intervening in some way.