If you are browsing Youtube videos on your phone and they appear blurry, then this is usually because of your connection speed.
If you are using 3G / 4G and you're in an area of low signal strength, then Youtube will sometimes automatically reduce the quality of the video. Unfortunately this can make the video very pixelated and of poor quality.
If you are using the standard Android browser, then when the video is in full screen, press the back button. Then press HQ and start playing it again.
If you are using another browser, then press the Settings icon on the bottom right of the Youtube video. Then change the quality.
This may improve the quality, but it may also lead to buffering issues.
Another solution is to use a WiFi connection, if one is available.