Check your project settings. In particular, the
Display > Window settings related to the size of the render target, stretch mode, stretch aspect, and stretch shrink.
It looks like Godot has been instructed to render your scene at a lower resolution and upscale it.
As you go about adjusting these settings back to their defaults to figure out what is causing the issue, I recommend you do this by rendering the game in windowed mode. Your issue could be as simple as thinking your full-screen resolution is set to something different than it is.