Is it my imagination, or have people become more stupid through the years?

Yes and no ..
Obviously we've advanced loads over the years indusry/technology wise ..
But I think that might actually be making people more stupid because they don't have to think for themselves anymore and spend half their day mindlessly staring at a screen .. people don't know how to interact with other people anymore, imo ..

People are stupid, yes.

It's mostly the media.
Watch a few commercials and a few family movies, kids shows and Comedy's from the past year, with an objective point of view, as in what kind of person would go for this or find it funny, and why would someone want people watching this.

Now understand that children are seeing and being influenced by this cheap garbage, now on a steady diet of mind numbing media that is literally designed to keep you/people stupid.

So yes that's part of the reason, the other is proper parenting, parents that rely on the system and schools to educate their children will often have stupid kids.
(No offense to any children)
It's not their fault and will turn towards knowledge if they were raised to seek it out.

So yes people for the most part are stupid, but they don't know any better.
All you can do is teach your own to seek out knowledge and don't rely on the system.
when you're child is kindergarten age teach them, it's as simple as teaching them to read.
teach them to read really good before first grade, and they will see the others that can't and will see the value of knowing and seek it out themselves.

What I see is an increase in assholery - the tendency of people to believe they are entitled to special privileges accompanied by total inability to see that other people have rights, and total indifference to their complaints.

That would explain most of your specific annoyances, and I guess you could call it "stupidity", but it's a special sort of ignorance.

Call me an old fuddy-duddy if you will, but I suspect the medium we're using now is largely to blame.

On the Internet, it's easy to start to believe that you are the center of the universe. For many people, their closest relationships aren't with their neighbors or the people they encounter regularly on the street or in their local store, they're mainly with people who live miles or even continents away. The "community" we belong to doesn't include the people we physically encounter, but rather consists of a constellation of dispersed people who have only one thing in common: they know us.

An anthropologist named Robin Dunbar suggested in the 1990s that humans are only capable of maintaining stable relationships with around 150 people. I suspect that once we've filled all those slots with family, close personal friends, and internet friends, everyone else tends to become an annoying distraction.

People don't have common sense, think they are entitled or think they can get away with being absolute smart alecks yes.

I'm that guy that has two items to buy in the self checkout lane, and I just can't F-ing get it right. 20 minutes later there's 15 people standing behind me, and six store employees trying to help me. On top of that, I've got Squinky my inflatable horse with me holding him under one arm, and trying to do everything else with the other. Squinky needs stuff too, mainly womens underwear and a bra or two. We get thru it eventually, the other customers don't do so well though :)

I think that you meant to ask, 'Is it my imagination, or have people become more stupid through the years?', to paraphrase.

The term 'dumb' means; an inability to speak, or mute, regardless of what the very stupid politically-correct dictionary people say, who are sometimes incorrect, and persuaded by particular groups of stupid people, to define things in an incorrect way, and /or deem such terms to be offensive, seemingly on a whim.

To answer your question, as paraphrased, above: Yes, people generally and gradually have become more stupid through the years.
The reason for this is: That because the world's geomagnetic field has been gradually weakening, people have been exposed to greater amounts of energies and particles of various astrophysical objects.
This causes dementia to develop in the human brain.
People have been gradually losing their ability for higher intellect and reasoning, as a result.

It is "common sense" that had caused many of the problems in society, that are clearly evident today.
"Common sense" dictates that one should follow and obey leaders and so-called authorities.
But the leaders and so-called authorities have become demented and corrupt, often acting impulsively.
People often do not recognize in themselves or others that they have become demented, until it's too late. They continue to follow the lead of demented people, because most of the followers have become demented themselves, and are unable to rationalize things properly.

Anyone who has even the slightest amount of reasoning ability left, would have fought against this very corrupt mob of I don't know what, from I don't know Nowhereland, of meaningless nothingness.

It doesn't matter much anymore, because there are not enough normal people left, to fix the damage that the gang of incompetent imbeciles had caused.
This entire civilization is on an irreversible course for an eventual catastrophic collapse, as a result.

Of course, in no part due to technology. It's gone berzerk. People are too fat and happy. Pretty soon, they'll have a little machine that makes them pancakes and rubs their balls. (*cough* not typing this from an iPhone at all *cough*)

Definitely, it began with getting rid of grammar schools, continued with mind numbingly boring reality shows and the media steering people into the vacuous celebrity culture.

Definitely.

Solid observation.
Fat slobs on their ifones.
You know it's shit when stores offer electric wheelchairs for fat slobs.
You see them in traffic camping out in the passing lane going slow.
Cell phone in one hand, cigarettes in one, crispy cream in their lap.
Totally oblivious to traffic.
Why can't I just nudge them of...
Yea, they're stupid

The IQ of the planet is a constant. The population is growing. So in answer to your question, yes.

Yes you're stupid.