pure data wise

Data-wise, the screen is 32x30 tiles, which is 256x240 pixels, or 280x240 including the border. (The height is set by the modified NTSC standard at 240p60, and the width of 256 was chosen to simplify 8-bit arithmetic, plus 24 pixels for a border.) With square pixels, the aspect ratio would be 16:15, or 7:6 including border. The video timing was chosen so that this fills the entire TV screen, which is 4:3. As a result, the pixels have an aspect ratio of (4:3)/(7:6)=8:7 (varies a little between TVs). However, the NES could only flip sprites and not rotate them 90°, so this could be taken into account when creating the rotated versions.

Another successful system with non-square pixels was the IBM PC, whose CGA and EGA cards had a 320x200 resolution (or multiples thereof in other modes), which resulted in PAR (4:3)/(8:5)=6:5. Square pixels first became available with VGA's hi-res mode (16 colors at 640x480), adopted by systems such as Windows 3.1 and TempleOS.