I think that the fact so many people are reacting with disbelief to the idea that people could feasibly support Trump for the reasons that subreddit claims to support him (will Make America Great (i.e. a world power that's respected rather than blamed for everything) Again, doesn't have any truck with Islam, doesn't have any truck with political correctness, and so on) shows that there is a large chunk of America, which apparently includes the mainstream politicians, that has fundamentally failed to understand that there are many people out there who aren't happy with the 'modern' or 'acceptable' view on many things, and you ignore their opinion as irrelevant and outdated at your peril.
The most interesting thing is that you can't paint these people as any one thing - they patently are not all racists, or morons, or any one thing. They are a rainbow coalition of those who feel ignored and frustrated by the mainstream of politics.
They simply reject the ideas they feel are forced upon them by politicians - for example, they don't want to accept Islam in their country, and feel justified in that because a form of Islam has brought many troubles to many parts of the world, and (in their eyes) the fact is that in Western Europe, integration of certain Muslims is proving very difficult because those certain Muslims hold to an idea of society that is not just very different, but almost totally incompatible with Western society, as it holds values that Western society has deemed unacceptable, like the treatment of women and homosexuals as inferior. These are seen as inarguable facts, and by ignoring them and refusing to have the debate, by painting anyone who voices their concern at these perceived facts as either a moron or a racist, the mainstream of politics has left many normal people, not just extremists, completely frustrated and disenfranchised.
In further examples, Trump supporters don't want to be part of a society that cannot handle ideas or thoughts deemed unacceptable by some unappealable general consensus, because they feel that stifles debate and is fundamentally un-American. They don't want to be part of a global trade network that they feel actively works against the interests of the many in America, now that they have seen the effects of free trade and what it does to the working class of America. They don't want to be seen as racist, or bigoted in any way for having these views, when the majority of them feel they are not - they feel they are merely making a sensible judgement based on the evidence that is presented to them.
By painting them all as racists and/or moronic bigots, their beliefs in the fundamental gap between them and the mainstream of politics are further reinforced rather than challenged. Of course, the fact is there are racists and bigots amongst Trump supporters, but they are not all racists and/or bigots, there's far too many of them for that to be even remotely close to true.
Most of all, Trump supporters want to believe that their voice and their vote matters, and that their leaders care about them. On the above mentioned issues, and most of all on making their people believe they are listened to and valued, mainstream politicians have failed spectacularly. Either by refusing to entertain the debate their people clearly feel is necessary, or through being clearly disinterested in normal people's lives and focusing on power and money in Washington, politicians have assumed their 'base' will always support them no matter how far from that base they move in reality.
Trump has sensed and seen this massive gap in the market (which, interestingly, is one of his biggest skills as a businessman - forcing himself into markets perceived as closed by spotting the gaps created by those very assumptions) and has exploited it to tremendous success, relative to what he was perceived as being able to achieve initially.
So ultimately, yes, r/The_Donald does represent Trumps supporters. It has a wide variety of posters, from people passionate about retaining American jobs in America and telling are trade to go fuck itself, to ironic meme-making students who think it's all hilarious, who all support Trump for a wide variety of reasons. But they are unified by a sense that no-one else in politics gives even a tiny shit what they think, and that that will not be allowed on their watch, and that together they can send a message that politics has to account for how they feel and think, because that is their right.