They want big man theory but with AOC and when that doesn't happen they get mad that AOC didn't fulfil their fantasy of participating in the workers vanguard

This is a strawman. Most people complaining are just tired of seeing praise heaped on imperialist social democrats because they're American and they claim to be socialists.

Yeah but the person above is arguing directly against that, saying no great man could save this.

Some do have that reasoning, and they are people being disillusioned from their liberalism. Others are tired of what they see as the praising of blatant opportunism.

I.e some want a great person to rise up and create the party and are upset when betrayed, and others are tired of seeing, "the Mensheviks are the way!"

But this topic doesn't just concern the "Great Man" when these debates pop up, as this often is the result of genuine organizating efforts. If a Mamdani administration doesn't do much to advance a socialist cause, harms it, etc. that is thousands who spent months of effort backing an opportunist cause instead of something that may actually advance struggle, and desiring to prevent that in anyway is obvious. Not every critic is productive of course, but it is something worth struggling over as it is a fundamental question of tactics.