The CloneStamp tool has not been conceived or designed as a tool to paint images. Its sole purpose is to correct small-scale defects, residuals or artifacts cosmetically, replacing them plausibly with nearby data. By small-scale I mean structures smaller than the mean PSF of the image over the region being corrected, or small artifacts whose distribution and geometrical properties make them easily identifiable and removable without compromising the integrity of significant data.
In general, other uses of CloneStamp to paint larger image (or mask) regions are, well, painting. When applied to astronomical images, these procedures are contrary to PixInsight's underlying philosophy of image processing. If you find yourself in the need of painting an image or a mask, then the problem is in your image processing strategy (or lack of thereof), not in the image or in the data. Depending on how and why you apply them, exactly the same happens with tools that replace existing image data with invented data, such as StarNet. The only real solution to these issues is learning more about image analysis and processing. There are many online and printed resources to achieve this goal, which have already been pointed out by other users.