The blur happens because of canvas fingerprinting protection. A canvas element has its dimensions specified in “content space” (in CSS pixels) and to render it properly, the website will ask the browser for the devicePixelRatio property, which indicates how many of the screen's actual pixels should be used to draw a single CSS pixel. When Mullvad Browser spoof the devicePixelRatio property, the canvas based elements can be rendered blurry if your screen's actual devicePixelRatio does not match the spoofed value.