Most people understand that. But what they rarely acknowledge is that deciding who's a good or bad president will always be a value judgement--a matter of opinion as much as fact. People act like there's an agreed upon standard for deciding what makes a president good or bad. There isn't. Whether you think a president, or any other politician, is good or bad depends on what you want that president to accomplish. It depends on what kind of country you want him (or her, eventually) to work toward. Answering "Who's a better president" isn't like answering "What's the temperature outside?" You can't just pull out a thermometer and take a reading. It's more like asking whether steak is better than lobster. It is--at least in part--a matter of opinion. It's a matter of what you value, and different Americans value different things. If you want to live in a country where the government has a strong presence and works toward more economic equality, you'll think someone like FDR is the model of a good president. If you want to live in a country with minimal government, where economic freedom is valued over economic equality, you'll think someone like Reagan was a good president. There's no law written across the sky that says exactly what constitutes a good president, or a good country.*
Don't get me wrong. I'm not saying there are no objective standards for judging these things. You can certainly measure unemployment or inflation, for example, though once again, it's not easy to say how much they depend on a president's actions. I'm not saying it's completely a matter of opinion, just that it's substantially a matter of opinion. It's weird that people don't acknowledge this. I'm also not advocating some kind of radical relativism here. I don't mean that what makes a leader or society good is nothing but a social construction. Mao, Stalin, and Hitler were bad leaders who oversaw bad things in their societies. Still, politics is to some extent a social construction--what constitutes a good country or president depends on what We The People want and value. The good society just doesn't objectively exist in the same way the deepest part of the ocean does, at least not in a way we know how to measure.
Besides, it's not like any of us are in a position to rule on these matters once and for all. We like to think our opinions are based on pure, clear-eyed reason, but for the most part they aren't. I suspect most political opinions are largely a matter of upbringing, socio-cultural identity, and innate temperament. Reason and evidence are bit players at best. How many people ever really engage in some kind of radical Cartesian doubt; setting aside every belief and trying to reason out from scratch which ones are strong enough to remain standing? Hardly anybody. Some may try, but it's not even clear that humans are capable of that kind of pure objective reasoning.
In short, judging societies, or policies, or politicians is at least as much a matter of opinion as a matter of fact, and our opinions aren't exactly based on superhuman rationality. We're only human, our politicians are only human, and our politics are only human. All too human. That doesn't mean we should just give up and conclude it's hopeless to try to muddle our way toward a better society. Maybe there are better ways of measuring the goodness of a society.** Maybe one day we'll come to an agreement about what kind of country a president should work toward, and then we'll have more objective way to measure whether she did a good job or not. Until then, maybe we should spend less time ridiculing the political judgements of others, and more time questioning our own. None of us has it all figured out yet.
*Or if there is, we haven't found a way to verify with enough certainty that everyone can agree on it.
**Some sort of national happiness index, like Bhutan has, seems promising to me, but I know people who would think that's crazy. The overall life satisfaction of a country's citizens does seem like a real, semi-measurable criteria by which to measure a government's success. Way better than GDP, anyway. Well, in my opinion.