He doesn’t always vote with the democrats, but if a republican had won that seat, they would be much more likely to side with the republicans.
He hasn’t done anything single-handedly, in every case where he gets blamed, it’s because there’s a huge Republican minority opposing something and he joins them. You can never blame him unless you’re first blaming all the Republicans he’s joining.
He’s a lightning rod because people think that for some reason he should vote with the democrats every time, but that’s not realistic. He’s a democrat from a very conservative state. The problem isn’t Manchin, it’s that they have to rely on Manchin and Sinema so much. If the democrats had a firm majority, his votes wouldn’t matter. Instead he’s often the deciding vote on something, so if he doesn’t support it it fails. But, again, it would be worse if a Republican had won instead. Right now the senate is 51/49, and the Democrats can occasionally pass things. If Manchin had lost it would be 50/50 and they’d virtually never pass anything.
When people look back at Biden’s term, they’ll note that it was the Republican who blocked everything, and that the 51/49 senate was too close to pass anything meaningful. Sinema and Manchin will probably get a mention, but anybody objective will note that it was the Republicans who blocked things, not Manchin and Sinema.