Let's take game development as one example. You can feel mediocre because you wrongfully compare your progress and success against outliers like Minecraft. However I don't think most people who want to be good at game development are ignorant of what an outlier is. It doesn't take much to learn how big the climb is once you start.
The real difficulty is that the tools have been democratized. The market is flooded with games from developers of all skill levels. Even the most niche games have so many options that most games have trouble distinguishing themselves from the competition. It can be disheartening to realize that your most unique and amazing idea is not unique or novel in most respects and that it will be competing in a pool of several thousand games with similar ideas.
The other difficult factor is that the greatest games make the appearance of being simple and effortless. We only see the end result... not the 200 games the developer made before releasing the one we noticed. The myth that genius is a gift you are born with can certainly make most of us feel mediocre.
That's even before we get to fear of failure, social pressure, and market pressure! Many people want to be good game developers but they also want to feed their family and give their kids a good education... so they only have so much time in their lives to dedicate to improving and developing. Even if they are full-time game developers many of the same fears apply.
It's the tyranny of being just smart enough to know how much you don't know. It can also be a comfort though if you're not worried about being the stand-out genius and just like doing good work.