Your question contains hints at where you're coming from.
At first, I started thinking that maybe some Lemmy users are really unhappy and find criticizing a something stronger helped them feel better
You look at the criticism and you think they're simply people being jealous or something, that's not it at all. To be fair I'm not sure about what criticism are you seeing constantly, but the ones I've encountered mostly reflect my own criticism of it, e.g. the worst health care in the world, terrible education, abismal class gap, blatant racism, police brutality, overspenditure in military, school shootings, etc, etc, etc. Let me be absolutely clear, the US is NOT stronger than any developed country in any of these, I've lived in places with terrible healthcare that I would not wish on my worst enemy and they're still leaps and bounds better than what you guys have.
But, we don't see them do that with other targets, just the US.
But we do, the difference is no one is making posts claiming Russia is a better country and that's why people criticize it. And most other countries accept their shortcomings, take it from the thread a while back from what people in the US need to hear and the similar one for Europe. Every single one of the answers on the US had someone saying that it was just jealousy, or that it wasn't all that bad, or that that was better than the alternative, or one guy even tried to tell me that his city had none of those issues. On the other hand in the EU one most of the answers were more akin to "you're right, this is something that's really bad here".
reject the US entirely. In other words, they may be Russian, Chinese, et al. agents working to feed a stream of propaganda in order to further cause chaos and lack of unity in the West as we have seen them do before.
Let me be extremely clear here, Fuck Russia, Fuck China. For all of my criticisms of the USA it is still a (very weird and indirect) democracy. So if I had to choose between those 3 I'll take the USA every day of the week and twice on Sunday. Luckily I don't, and I can live in a country without any of the issues I see there and express my opinion on how poorly the USA treats their citizens (you have to keep paying taxes after moving abroad for crying out loud) in the same manner I would express myself against any other country doing the same.
Criticize something obviously unjust that the US has done. Ignore that all other major powers have also committed atrocities.
Most of the criticism I see re for what the US is doing NOW, not in the past.
Paint the US entirely as evil so that nothing the US can do is just
I don't think that's what the majority is doing, there could be a few people like it, but for the major part I think people recognize that the USA is a a "decent" place to live, especially when compared to Russia or China. But what you need to understand is that it's not the greatest country of all like you think, most of us would not move there unless we were offered ludicrous amounts of money.
EU and Russia on the same team lol
No one in Europe believes that, in fact you can still see plenty of Ukrainian flags around.
West fragments as US and EU relationship dwindle from pseudo-populist movements (e.g. MAGA)
If you think MAGA is pseudo-populism I would hate to see what you consider as real populism.
The short answer is that you're probably not used to people telling you the US is not all that great, so you see that as an attack, and you think that because people criticize you that means they are trying to make others hate you, but I think most of us are just pointing your shortcomings because a lot of you act like very entitled narcissists who think there's nothing wrong with the USA so obviously those who speak against it must be puppets.
TL;DR: criticism of the USA doesn't mean there's nothing good there, much less is it an endorsing Russia or China. But you don't need to tell narcissists what they're doing well, they'll tell you themselves.