Leftists were broadly shockingly happy with Biden until Gaza, although dissatisfaction with his immigration policy was mounting. But on economic policy, people were unironically calling him the best president of our lifetime.
The actual deal with Mamdani is that *leftists are emotionally invested in Mamdani's success* so when he does very banal incremental things they get happy while when the Biden administration did much bigger things they didn't care. But you can just open your heart to meliorism in general.