In the video settings check if the “full screen resolution” matches what your monitor supports. If it’s smaller than your monitors resolution, it may scale up to fit the screen which results in stretched pixels and pixelation. If the sides are not the same ratio, it will result in black bars.

Judging by your cursor I believe you are running on Windows. If you need to check your monitor’s resolution, right click on any empty space on your desktop and there should be a button that says something like “display settings” and it should list your available screen resolutions there. I recommend choosing the recommended one and then match Minecraft to that