As with almost any such blanket statement, it's trivially simple to prove this one false in specific cases, for specific groups. As an example, the monoculture of yeast that went into a cold tasty frothy beverage resulted in an excellent refreshment. You will note, however, that it didn't work out so well for the yeast. They're all dead. So, let me say more specifically that monoculture is usually bad for the population to which it applies. The fundamental reason that monoculture is bad is because a single flaw is necessarily endemic to the population. A fatal flaw will result in extinction.
The problem is that monoculture is so cursedly alluring. On the grand scale, it's so very tempting to attempt to design a perfect society. The problem with that is that perfection is so very hard to achieve. On the smaller scale, the problem centers around the desire to do what's best for oneself. In general, this tendency helps society and individuals to flourish and grow. The problem arises when society is confronted with complex issues. Often, these issues are complex enough that it requires years of study and a fairly keen mind to understand them sufficiently well to make a good decision. In such circumstances, it's entirely reasonable to defer the decision to a trusted expert who has a keen mind, and who has spent the requisite years of study in the field. However, for a complex enough subject, experts sometimes disagree. If you have enough knowledge to know which expert to believe, then you probably don't need an expert in the first place. Thus the next place to turn is a consensus of experts. Now, don't take this to imply that a consensus of experts is necessarily wrong. Chances are, they're the best people to listen to (depending, of course, on the subject). The problem is that the consensus of experts can be wrong, and if they are universally followed, then any fallibility becomes hugely magnified. If 60% of doctors think that drug X will save the lives of their patients, and prescribes drug X for them, then if it turns out that drug X is deadly, 60% of the population will die. That's an unthinkable tragedy. If, on the other hand, the AMA listens to those 60%, bases policy on that consensus, and causes 100% of doctors to prescribe drug X to their patients in accorance with their guidelines, then if that drug turns out to be fatal, it means the extinction of the entire population. That's worse. This raises the stakes of evolution from the individual level to the societal level. If an individual makes a bad choice and dies from it, the species improves as a result. If a monocultural society makes a bad choice that results in its extinction then, well, it depends on what the universe is made up of. If the Earth is all there is, then if humanity extinguishes itself then maybe the cockroaches will have better luck. If life is endemic, then perhaps the little green men on Epsilon Eridani will make better choices. If the Deists are right and God is more of a clockmaker, then perhaps someday He'll wander back by, look at the Earth, sigh and wind us up again.
It is for this reason that I am of the political mindset that I am, which I often find seems to be confusing to so many others. I'm strongly in favor of diversity, though not so much the kind that people display by having different skin colors, wearing different clothes, and speaking different languages. I mean the sort of diversity where people live under different laws. This puts me at odds with the liberals who want the nation or even the world to conform to their view of enforced universal charity, as well as the conservatives who want the nation or world to conform to their view of enforced universal morality. While this often puts me solidly in the camp of the libertarians, even that doesn't quite fit. This is because of two simple yet humbling truths: First off, as much as I love freedom, I am forced to recognize that there do exist people who do not want to be free. The second is that while I strongly believe that freedom is the best way for society to progress, I am humble enough to admit that I may be wrong. For this reason, I am strongly in favor of a diversity of laws, and there is only one freedom that I feel should be completely inviolable. That freedom is the freedom to move from a society whose laws you do not agree with to a society whose laws you find more comfortable.
This puts me mostly in the company of the constitutionalists. The most important part of the constitution is that tenth amendment, which states, for those of you who may have forgotten "The powers not delegated to the United States by the Constitution, nor prohibited by it to the states, are reserved to the states respectively, or to the people." This means that, with 50 states and binary issues, it is possible for every single American to live in a state whose laws precisely reflect their opinion on at least 5 issues. If the states delegate some issues to the respective counties, and the counties to the cities, and the cities to the neighborhoods, and the neighborhoods to the families and individuals, then it's possible to imagine a nation where everyone is happy with their laws.
This of course does not take into account those people for whom it is important to prevent everyone from doing those things that they find distasteful. A sufficient number of those who wish to impose their laws on others (absent, of course, a monoculture of such people) will always result in some dissapointment.
But it gets better. If there is freedom of movement, and freedom to choose your own laws, then the society will have a tendency to separate, and form a diversity of monocultures, where people not only live under a set of laws that they are happy with, but live amongst people who are like-minded. It doesn't really solve the problem of consensus extinction, but it does at least give the chance that those who reject the consensus will have a place to go where their quackery is accepted. If that quackery turns out to be right, then their corner of society may be all that's left. If not, then we have the chance to see evolution in action.