A Distant Mirror, as many reviewers of the time, and indeed Tuchman herself noted, was rooted in her assumption that society in her own time was breaking down. Tuchman views the period as maked by senseless violence, duplicity, and ignorance. She does not engage with much of the scholarship of the period, and, as many historians of medieval history noted (see the American Historical Review vol 84, No. 3 if you can get access for a famously scathing assessment by medievalist Bernard Bachrach) doesn't seem to have much of an idea of what made medieval people act as they did.
As for the emergence from this period of barbarism, as she portrays it, I thoroughly agree with pohnpei's assessment: she doesn't seem to have much of an idea of why this happens, other than her often-repeated faith that history just moves, and sometimes "ideas" just break out of "molds." She's not interested in thinking about causality, just in using the fourteenth century as a "mirror" of her own time. There are lots of problems there. That said, I have used this book in European history courses as a way to disabuse students of any notions of "knights in shining armor" that they might have, but I wouldn't use it as an example of how to do history. Guns of August, on the other hand? Different story.