When downloading icons, paying attention to their file format and their size is important for the icon to look good.
If you are downloading icons in PNG and they look blurry or pixellated, try downloading the largest size instead (512x512 px).
If you want the icon in a larger size, download the SVG format instead. The SVG (vector) allows you to resize the icon infinitely without it losing quality.
For more detailed information about file formats, please check What icon format is suitable for your intended use.