I'm using Gimp 2.10 and when I paste an image over another image and transform it, the clarity of the pasted image decreases dramatically. I'm completely new to all of this and am learning as I go. Any help is appreciated. I attached an example, notice how the screenshot of the app is somewhat blurry.
In Gimp (as in all raster image editors, and this includes Photoshop), what counts is the size in pixels. If you are enlarging the image to fit the target, then it will get blurry. Even with a pasted image of an adequate size, if you have to rotate and tweak the perspective, it will also induce a slight blur.
You can mitigate some of this by using
Filters>Enhance>Sharpen (after making a selection around the pasted image using the lasso):