You are stretching a low resolution to fit a high-res monitor. Fix: Ensure your GPU scaling settings are set to "No scaling" or "Aspect ratio" (centered).
Note: Custom resolutions work best in or Windowed mode. If you set Fullscreen = 1 , your GPU driver will likely stretch the image. osu custom resolution
This question pops up in the osu! community frequently. The answer is a definitive You are stretching a low resolution to fit