Welcome to the forum!
I'm not sure if I can help. But I am curious why copying to the clipboard affects the quality (or why you think it does)? To my understanding the clipboard should have no effect on the image. But let me ask some questions.
When you copy something from Inkscape, is it all vector contents? When you drew the image, did you either import any raster images, or use Edit menu > Make a Bitmap Copy? If not, then you should have all vector contents.
So the next question would be what happens when you paste into Powerpoint? I'm not familiar with that (although I probably should be more familiar) so I don't know if it supports either SVG or vector contents in general. I kind of have a notion that it should be able to support vector images.
If it does (then resolution is irrelevant, and) you should be seeing nice crisp images. If it doesn't, then it's probably been converted to some raster format, which means it's susceptible to pixelation, if it's scaled. It could be scaled automatically, if the native dpi of Powerpoint is different from Inkscape. That's the most likely explanation I can think of.
The grid resolution (I guess you mean how many lines and cells in a grid?) doesn't change the resolution of the image. It just configures the grid.
Changing the resolution for Make a Bitmap Copy won't do anything, if you don't actually use Make a Bitmap Copy. (Maybe you did, I'm not sure.) But if the problem is that Powerpoint and Inkscape have different native resolutions, then using that option would be the best way to fix the problem and continue using copy/paste. You'd have to find out what Powerpoint's native resolution is.
I don't even have Powerpoint (....I don't think....) but if you can attach a sample SVG file with an image that has this problem, then someone who has Powerpoint can do a test, and find out if that's the problem, or discover what the problem is otherwise. Also, a screenshot showing the poor quality that you see in Powerpoint would be helpful too (so we can confirm whether we've reproduced the problem).
(If you can't use an actual image from your project, then any kind of sample image would work, as long as you've confirmed that it has the same problem when you paste into PP.)
