This “block party” was a bust. Scores of Zohran Mamdani fans who braved freezing temperatures to celebrate the new mayor as he was publicly sworn into office Thursday were left disappointed by the bash the socialist pol had promised. Around 10,000 supporters stood outside City Hall during the event — billed as an “Inauguration for a New Era Block Party” by Mamdani’s staff — crammed into several barricaded pens without access to bathrooms or any food concession stands. “It’s definitely not a block party,” said Danny Mahabir, 30, an Astoria resident who told The Post he’d been expecting a mix...