Does the average citizen of the Empire know that Stormtroopers are clones? Secondarily, does this effect their attitudes towards them in any way?
In terms of timeframe, let's consider the entire lifespan of the Empire, from its birth in Episode III through its end in VI.
Logic would state that yes, the average citizen knew that many stormtroopers were* clones.
Exactly what percentage of current Stormtroopers were clones after The Empire was officially formed is up for discussion.
Consider the fact that there was a huge conflict actually called "The Clone War".
Dubbed this by Yoda at the Battle of Geonosis,
"Victory? Victory, you say? Master Obi-Wan, not victory. The shroud of the dark side has fallen. Begun, the Clone War has."—Yoda
Wookieepedia elaborates:
The war was named after the clone troopers utilized by the Republic against the battle droid forces of the Separatists. These armies, the Grand Army of the Republic and the Separatist Droid Army, were two of the largest ever pitted against each other in galactic history, and the fighting between them rapidly spread to countless inhabited worlds.
This was not some small skirmish in a remote area. It spanned across the galaxy. Those who were not involved were surely at least aware of it. Even if one was not aware of the specifics of the war, the name of it is sort of a dead giveaway that clones are somehow involved.
That's well and good, but what about 20 years later? Even the Force has largely faded into obscurity.
Even Luke, who lives on the planet furthest away from the bright center of the universe, knows of The Clone Wars.
BEN: That's what your uncle told you. He didn't hold with your father's ideals. Thought he should have stayed here and not gotten involved.
LUKE: You fought in the Clone Wars?
BEN: Yes, I was once a Jedi Knight the same as your father.
So, sticking with G-Canon - while not 100% positively affirming it, there is enough evidence to fairly assume that the average citizen knew that Stormtroopers were clones.
The stormtroopers are not clones but are instead volunteers.
Out-of-universe, this has been confirmed by Pablo Hidalgo of the Lucasfilm Story Group in charge of canon. The explanation given is that the production of clones has been phased out, and since the clones age at twice the rate of normal humans they are too old to serve as stormtroopers. They have been replaced with non-clone volunteers who are patriotic and loyal to the Empire. Video evidence can be found on Youtube (starting at about 2:56 into the video).
In-universe, this is confirmed by the canon novel Tarkin. The novel includes an incident where a group of stormtroopers are seen by Moff Tarkin without their helmets. The stormtroopers are led by a Kamino clone sergeant but all the other troopers are non-clone recruits. Here is the relevant quote from p. 94 (the stormtroopers are loading Darth Vader's meditation chamber onto Tarkin's ship):
When the stormtrooper operating the equipment accidentally allowed the flattened sphere to bang against the edge of the cargo hold’s retracted hatch, Vader stamped forward with his gloved hands clenched.
“I warned you to be careful!” he shouted up at the trooper.
“My apologies, Lord Vader. Wind shear from—”
“Excuses won’t suffice, Sergeant Crest,” Vader cut him off. “Perhaps you are aging too quickly to remain on active duty.”
Tarkin couldn’t make sense of the remark until he realized that Crest’s was a face he had seen countless times during the war—the face of an original Kamino clone trooper. The bare-headed others comprising Vader’s squad were human regulars who had enlisted after the war.
The novel takes place about 5 years after the end of the Clone Wars, so clone production evidently stopped at the end of the Clone Wars and within 5 years the clones were becoming too old to be useful as soldiers. Hence, they were replaced by non-clone volunteers.
Since the clones are recruited from volunteers it would be widely known that the stormtroopers are not clones.
