It is possible to tilt the camera using viewRectangle?
Not directly, but you can easily make a BoundingSphere from the rectangle and then use viewBoundingSphere to do what you want. In this case, you can modify pitch for your desired angle and heading if you want to view it from an angle other than north.
var heading = 0;
var pitch = -Math.PI/4;
var bs = Cesium.BoundingSphere.fromRectangle3D(rectangle);
viewer.camera.viewBoundingSphere(bs, new Cesium.HeadingPitchRange(heading, pitch));