The expression "If you can dream it, you can do it!" is wrongly attributed to Walt Disney, but it does encapsulate a very real part of his outlook, and of the American outlook more generally, and it must be admitted that when used in moderation, this perspective can have wonderful results. The proper understanding of this expression is as a hyperbole intended to free us from the shackles of stupefying habit. It becomes a serious problem, though, when it is taken as a simple, unvarnished truth.
For one thing, it is clearly impossible for it to be true for everyone, because one person's dreams often contradict another person's dreams. Jeb Bush dreamed of winning the US presidency in this year's election, for example, but he could not do it because enough other people had different dreams. Likewise, some things are simply physically impossible. The character Superman is after all the embodiment of the dreams of his creators and developers, but most of what he does is absolute nonsense.
It is a good thing that "If you can dream it, you can do it!" is false. For one thing, it is good that our dreams are not limited to the things we can do -- "a man's reach should exceed his grasp." Much more importantly, though, anyone who is able to look at himself at least somewhat honestly knows that absolute power would corrupt him absolutely, destroying his character, making him an enormous threat to others, and ultimately making him miserable. This has been treated many times in fiction -- notably in the Star Trek episodes "Where No Man Has Gone Before" and "Charlie X", the Twilight Zone episode "It's a Good Life", and even the Ring of Sauron in Lord of the Rings.
Although we are not actually omnipotent -- and that is a good thing -- there remain two possibilities for those who cling to the idea that "if you can dream it, you can do it": we can either prefer to live in a world of "pure imagination" in preference to reality, or we can reject the existence reality altogether and claim that we are compelled to construct our own "realities". The latter approach has grown in popularity and seems to be the unquestioned metaphysical position of the millennial generation. It is completely ubiquitous in society today; it is even the current metaphysical framework of our legal system, as exemplified by the "sweet mystery of life" passage of Planned Parenthood vs. Casey. America seems destined to be destroyed not by the bang of Armageddon, but by the whimper of Weirdmageddon.
This situation is not unique -- after all, it is just another version of the Serpent's ancient lie that "you will be like God", in this case creating our own "realities" -- but it is exceptional. In another age, the fact that reality is better than unreality was considered so obvious and important that it motivated the center of the Ontological Argument for the existence of God. (The fact that even most believers in God think the Ontological Argument is flawed does not negate the importance earlier generations placed on reality.)
Astonishingly, a good commentary on all this, and the origin of the term "Weirdmageddon", can be found in the cartoon Gravity Falls, which was produced by none other than the Walt Disney corporation. Before I go any further, there are spoilers ahead.
The main antagonist of the cartoon is a strange "demon" named Bill Cypher. The first time in the show's narrative he is summoned, he exits saying, "Remember: Reality is an illusion -- the universe is a hologram -- buy gold -- bye!" His whole motivation is a rejection of reality in favor of what he considers fun. If that means that "[he's] got some children [he] need[s] to make into corpses", well, so be it. He really is the perfect representation of the Zeitgeist. The fact that "he" appears on the one dollar bill makes the correspondence all the better.
In order to defeat him, the main characters had to give up their pride and selfishness, which after all are the origins of this war against reality. Even more dramatically, Mable Pines is captured by Bill Cypher and placed in "a world of pure imagination" apparently suited to her tastes -- though in reality, it is made up of disgusting and malevolent worms that remain disguised until the illusions they create are confronted. Her twin brother Dipper goes in after her to find his own hopes falsely offered to him, including the opportunity for romance with Wendy, the girl with whom he is infatuated. He sees through the deception and rejects it. In the end, both twins have to explicitly accept that the real world, with all its flaws, is better than an "ideal" world of imagination and illusion.
Gravity Falls would have been an entertaining program even without its relevance, but it turns out to be astonishingly on target, much more so than Walt Disney Inc. can have really wanted or realized, though I have to suspect it was somewhere in the back of the mind of Alex Hirsch, its creator. I have to credit him with deliberate irony in having Disney pay for and distribute a cartoon that celebrates reality over the lies that the corporation cherishes so much.