What size did you render this at? The one you have here doesn’t seem to be that large, so that could easily explain the pixelation you’re talking about.
If you zoom any digital image far enough you will begin to see pixelation. Make sure that the render setting is set for 100%. Did you set the render seed to change for every frame? If not you can enter #frame into the seed and it will change the seed for every frame. This helps with the animation not looking strange.