Okay, describing visual minutia with words is difficult. Here's a handy picture, using the display resolution of the 3DS.
On the left is the image being displayed in the original aspect ratio of an NES game, with black bars on the side.
On the right is the same image stretched to fill the entire 3DS screen.
First a familiar NES screenshot, then a couple images to illustrate why stretching is bad.

And yes, my font choices were
entirely biased.