Even Rex's taunt supports it:
Rex: "I hope you have a better class of soldier than those 'Stormtroopers'..."
Kallus: "They serve the Empire Well... And I have a Great Many of them... "
That's what i'm saying though. The new canon is beginning to portray them that way and i'm staunchly against that. However the statement could easily be translated as Kallus having a disregard for his men's lives.
No fighting force in history has ever successfully conquered and maintained a massive Empire with vast armies of basically civilians in armor with guns and next to no training. What many empires did was have a core of highly trained or experienced troops supplemented by inferior troops that would either be used as cannon fodder or to bulk up numbers. Colonial Empires supplemented their armies with local conscripts and recruits that would help enforce colonial rule over distant territories. Why would the Galactic Empire be any different?
At this point in the new canon Stormtroopers might as well be battle droids but with the large downside you have to feed them, appease their needs and desires in order to keep order amongst the ranks, and replace them as they age out. Having to supply an organic army of poorly trained troops is incredibly stupid. If you're going organic then you train your troops well.
I just got finished reading the Thrawn Trilogy and in that Zahn gave I think the best portrayal of the Imperial Forces in any SW book or spinoff. Stormtroopers were highly trained and indoctrinated troops that supplemented the regular army troopers. The regular army acted in the way stormtroopers are now. Cannon fodder that are unarmored conscripts largely that garrison Imperial held worlds and maintained order. The stormtroopers are more specialized and while not commandos were more of a threat.
More so then anything it makes the Empire exactly the same as the orc armies from Lord of the Rings and every other franchise from LOTR to Marvel to GI Joe where the bad guys basically rely on high numbers to overwhelm the hyper specialized or experienced heroes. It's not unique and not story telling I enjoy. I don't mean to derail the thread from the show but Rebels is guilty of making the Imperials insanely idiotic and i'm just not a fan. I like the show a bunch don't get me wrong but its one of its many problems.
By and large the majority of media from both old and new canon does not support stormtroopers being an elite fighting force. Books, games, and the movies show them as little more than canon fodder, but have the advantage of numbers. There are far more examples of stormtroopers being disposable military units with cool armor and toys rather than them being the elite fighting forces a few books make them out to be.
Does Rebels make stormtroopers a bit too useless? Yes. But they're far more effective than battle droids. They actively try to avoid danger, you see them actually use their heads (from time to time), and they've even pulled off some awesome stunts (the one trooper leaping into the tank and fighting off Kanan when the Ghost crew tried to escape with the Rodian, or the scout trooper on Ryloth that did the cool reverse maneuver into the Ghost ).
Also, you're trying to apply real world logic to Star Wars. That never works. Star Wars works by its own logic.