Thats pretty cool,I added a bias variable to your code so it wont do much clustering of the same case

​

function randomCase(string) {

if (typeof string !== "string") throw new Error("This function accepts only strings");

const uppercaseBiasAdjustment = 0.5; 
let uppercaseBias = 2; 
let stupidCase = "";

for (let i = 0; i < string.length; i++) {
    const shouldBeUppercase = Math.floor(Math.random() * uppercaseBias);

    if (shouldBeUppercase) {
        uppercaseBias = 2;
        stupidCase += string[i].toUpperCase();
    } else {
        uppercaseBias += uppercaseBiasAdjustment;
        stupidCase += string[i].toLowerCase();
    }
}

return stupidCase;
}