I do not see a clear difference between the four pictures besides the fact that the first two pictures are a different resolution than the last two pictures.
I would say that the problem cannot be shown with a screenshot if this is the worse screenshot you have.
Have you tried changing the look of your desktop?
Have you tried adjusting the settings of the monitor? There is usually a buttons at the bottom right corner of the monitor. One will be a menu button that will allow you to change the default settings or return the settings to default.
Check and verify the resolution for the laptop and the monitor. My laptop (17 inch) is set for 1920x1080 (1080p) on both. The monitor and laptop both default to that resolution. Other than size (17 inch vs 24 inch) the resolution and quality is the same.
On your screen shots I see that the monitor is not as sharp as the laptop, and I would guess you may be using a 15 inch laptop. It is quite possible that the laptop resolution may be less than the monitor default resolution so with mirroring it has to stretch the image to fit a larger screen at lower resolution.
As I noted previously, mirroring might be the issue. The display on the laptop screen is only 1366x768 and when that is stretched to fill the larger monitor at 1920x1080 it will lose quality/sharpness. The smaller display only has about 71% of the pixels in the larger display.
Try setting the displays to be side by side and see if that makes a difference. In side by side mode they each should have their own resolution displayed. I inferred that your earlier snapshots were done with them in mirroring mode since the data shown was identical…