Only Mariah Carey can arrive at an ice-cream date like a true diva, wearing thigh-high boots, fishnets and a corset. Spies tell us the singer kept several guests waiting at a table at kid-favorite Upper East Side restaurant Serendipity 3 for nearly two hours on Sunday until she stylishly sashayed in, wearing a short corseted dress and steered by two giant bodyguards. After their meal, Carey’s twins, Monroe and Moroccan Scott, joined the group with a nanny, and Carey ordered the “biggest sundae ever” for her kids. She took pictures of the pair eating the sweet treat, then tipped the waiter 200 percent of the bill.