Milkshake Duck
Someone who gains sudden fame for something nice and positive, only to soon after be revealed as a deeply flawed character with terrible opinions and/or a shady past, often involving corrosive social/political ideologies, which quickly tarnishes their fame and the good will people momentarily had towards them.
“The whole internet loves Milkshake Duck, a lovely duck that drinks milkshakes! *5 seconds later* We regret to inform you the duck is racist” -@pixelatedboat via Twitter