People are cheering on Iran out of pure spite, not out of genuine support for Iran.
The American people are tired of our government spending out tax dollars to support a genocide and then gaslighting us about it when we call them out. They are also persecuting dissenters on it. This has happened under both Biden and Trump, though Trump has leaned more on the persecution. We are fucking sick of it.
There is satisfaction in seeing someone stand up to genocidal Israel, even if the party doing so is despicable in their own right. I think people would greatly prefer getting behind a western democracy taking a stand, but we're at a point where people will take what they can get. Especially with it being clear that the aforementioned western democracies aren't going to do shit against genocidal Israel.