Is Super User unfriendly to beginners?
It's becoming more so, yes, but originally it really wasn't. As members with various attitudes come and go, the general tone of the site changes. It's hard to get people to change, but by adding or removing active members (whether by force, or they just go inactive), the nature of the site and the way it's perceived can also change.
What I'm trying to say is that it is difficult to make generalizations about the site, especially without bounding those generalizations by a fairly tight time window ("Has Super User been unfriendly to beginners over the past few weeks?" is easier to generalize about than your original question.)
Another point to consider: In general, "bad actors" (those who are unnecessarily rude) are usually in the minority. So, instead of referring to the site as a whole, if we focus on the minority of users who are "problematic", it might be easier to eliminate the problem by banning or warning the worst of the bunch (the most frequent/severe "serial offenders", as it were).
How can we help beginners understand things?
As someone who's written long, explanatory answers treating an advanced topic for a beginner, I know how hard it is to write good answers for beginners. It's extremely hard. You have to basically fill in all the knowledge gaps and assumptions and industry knowledge that they lack, and then answer their question.
And then after you've spent 2 hours writing an answer, it's entirely possible that someone else already beat you to the punch with an oversimplified answer that isn't as good, but gives them a "cut and dried" answer that they're looking for, so they may not even bother to read your long-winded answer.
Questions related to computing can often be very brief between two experts in the field who already have a large body of knowledge accumulated between them, but would require an enormous amount of explanation to properly explain to a less experienced or knowledgeable person. This is just the nature of the beast.
The fact that you are self-taught suggests (and this may or may not be true) that you are lacking many of the fundamentals that would be required in order for you to be able to answer your questions on your own, without needing to ask someone else. This might be why others perceive your questions as "bad" and are thus tempted to be snarky.
For example. Assume that someone does not understand the concept of fire. They ask you, "Is it possible to light a desk on fire?" Patiently, you explain that, yes, desks are made of wood, and wood can catch on fire if it gets hot enough.
Then the next day they ask you, "Is it possible to light tires (tyres for you Brits) on fire?" Again, you explain, with some irritation this time, that tires, when they get hot enough, can catch fire.
On the third day, they ask you, "Is it possible to light iron on fire?" Finally, you just throw your hands up and say, "If something gets hot enough, and it's inside an atmosphere with a reactant that can cause it to combust, ..." and you start giving all the conditions under which something can be lit on fire.
Once you're done explaining your answer about when things can be lit on fire, the person asks you, "What's an element?"
If you're a chemist or physicist, by this point, you're just wanting to tell this person to go back to college and take a few physics or chemistry classes. You're tired of explaining to them how the world works.
This is how many of us in the IT industry feel, because we have to, on a daily basis, deal with people who are (through no fault of their own) extremely ignorant of technology in general, and are nonetheless impelled or compelled to use computers to accomplish tasks in their life.
To be honest, it can wear away at you after a while, having to explain the same things over and over. It requires a lot more energy and consideration to give a thoughtful answer that will fill in all the knowledge gaps of the person; and it requires a great deal of patience to handle the many follow-on questions that are likely to come from it (assuming, of course, that the person doesn't balk at the complexity of the situation and say, "I give up!" or say "Thanks!" and then not even read your answer).
When you're speaking to another experienced person who has the fundamentals, you can give a quick off-the-cuff answer, and they'll get your meaning, and there will be no further questions. It's much tidier and easier. They may even ask a question that you yourself do not entirely know the answer to, and thus challenge you to go out and do some R&D and find the answer for them. These are the kinds of questions that technologists really enjoy answering, because they challenge us to learn more ourselves.
Most of the questions that I bother answering on Super User are questions that I found interesting or challenging. I've tried to improve my behavior and demeanor over the years by simply passing over questions that I find too basic or too annoying to answer – though on a few occasions I've taken a shot at an "epic" answer and occasionally receive kudos for it.
The only thing we can really do without banning people outright is try to convince those who are actively snarky/rude on a regular basis to please either (A) tone it down, or (B) simply ignore questions that you don't like; pretend they're not even there and move on with life. But then, how do you get that message out to the people who need to hear it?
I feel like I've written a meta answer almost exactly like this one, to a meta question almost exactly like yours, not too long ago. I guess you're not alone in thinking this. But I really don't have any good ideas for how we can constructively approach the problem in an attempt to solve it. Awareness of the problem is not sufficient for dealing with it.