Ninja's monitor has a 1080p resolution, which isn't the sharpest monitor you can buy. But with lower resolutions, your computer's graphics card doesn't have to work as hard, and it can push more frames per second. As a result, you can get smoother performance with lower resolutions.
Ninja gets between the the 70 and 220 frames per second with his PC, which is on the high-end of smoothness.
At the same time, monitors with higher resolutions like 1440p can deliver sharper details than a 1080p monitor, which can help you spot enemies from further away. The trade-off is that the game may not play as smoothly as it would on a 1080p monitor, as the graphics card has to work harder to render graphics at 1440p resolution.
4K monitors would offer the sharpest details, but few PCs — if any — can play "Fortnite" at higher than 75 frames per second. That's smooth enough for most players, but if you want Ninja's smoothness, you'll need a monitor with lower resolution.