I saw that easing variable was added, but no easing code was included so that easing happened. so, I modified the first few lines of function draw () to make easing happen:
function draw() {
var targetX = mouseX;
x += (targetX - x) * easing;
var neckY = -1 * (bodyHeight + neckHeight + radius);
background(204);
translate(x, y); // Move all to (mouseX, y)