MP3 is just the format, depending on the encoder and the settings you can go from "heavily distorted crap" to "a human being can't tell it apart from a lossless source".
YouTube does AAC, and for 720p or better streams the bitrate is 192kbps audio, which is good. If you're going to rip music from YouTube videos, try to get it from 720p or better sources.
Of course, this all goes to shit if the uploader used a crappy MP3 to make a high res/bitrate video, aka fake HD.
Hm, so I just looked it up again, and apparently YouTube stores 192kbps audio for HD videos, but the one that's actually played on youtube.com is 128kbps. So make sure whatever tool you're using to download the videos before you transcode them is getting the highest quality version. You can use this tool to find out what qualities are available and download them: http://www.h3xed.com/blogmedia/youtube-info.php