Hello. I have (now had) the same issue.
There is a solution, but it is very weird:
1) Go to
draw.io as usually to edit your diagram
2) Go to menu: View --> Page scale
3) Change the sacele to much higher than 100% ( for my 15.6 inches, 4K screen I had to use 400%)
4) Now make everything as usual but make everything significantly bigger (scale up your diagram, edit font size, line thickness, etc.)
5) Return to your Google Docs and update the diagram in document (select your diagram , than go to Menu: Add-ons -> Draw.io Diagrams -> Update Selected)
After that it will be clear and sharp in your Google Docs. (You can compare the attached screenshots to see the difference).
I agree it is weird approach, but it helped me. Hope somebody could find the better way.