Depends on the mp3, the source and chain to your headphones. Even an "average" CD player is probably going to sound better than a low bitrate mp3 out of computer or laptop with a mediocre source and amp (or no amp).
Thanks for the replies. I guess, in the case of the laptop, through the USB with a DAC is better than out of the soundcard.
In the case of the CD player, what is the most important feature that can make one player superior to another?