I don't know that Mamdani's win was *that* impressive in the grand scheme of things from a numbers perspective, but in his defense, he faced an absolutely nasty campaign and that had every institutional lever imaginable pulled against him. From the millions of elite dollars thrown at him to slanted and, frankly, unfair coverage of his campaign in the New York Times and other media outlets. And Cuomo, the slug that he is, is still a big name that registers with low-info voters and probably earned some votes on name recognition alone.
He's not really my cup of tea, I have my disagreements with him, but in that context, I think his win was solid (if not impressive)
The bigger takeaway for me from him winning along with candidates like Spanberger/Sherrill is that the Democratic Party is a popular front / big tent party and that it's power comes when it is unified. And all three shared a message centered around cost-of-living issues and other issues that voters care about in this moment.