Eight-foot-high piles of rat-infested trash are choking the streets around Gracie Mansion — while Hizzoner’s new home has gotten the white-glove shoveling treatment. Mamdani has meanwhile crowed that he can’t “imagine how it could get better’’ in the city, even as more and more New Yorkers are blasting the lack of “collectivism” in the Big Apple. The mayor’s Upper East Side neighbors are being forced to trudge through garbage-plagued streets, roaming rodents and mounds of snow tainted with dog pee a full week after Winter Storm Fern.