Is there a way to increase the quality of the screenshots that are taken by the default Screenshot application, the pictures have a very low quality especially when they are zoomed. I have a screen resolution of 1366x768
You can not tweak the quality of your screenshot, it only captures your screen or region (as is, the only quality adjustement you can use is the filetype used when you save your document).
If you need to modify your screenshot, you should use additional softwares such as Gimp or Krita
(I do not see your issue in the provided capture.)