Okay, so I've recently gotten a new laptop and set it up with my drawing tablet. I used krita with my older one, and I had a similar bug, but I could usually get rid of it by just restarting my device. With this new laptop however, I can't seem to get rid of it. Any lines I draw have a strange, jagged look and are never straight or smooth. Additionally, the mouse appears (when I use the tablet pen only) as a weird diamond that makes you hold it down in order to grab anything?? At first I thought it might've just been my tablet, but the jaggedy lines do the same thing when I draw with a mouse. I also downloaded krita on my school laptop to see if the problem was the same, and it was! I can't seem to get rid of those jagged lines, and even if I restart the devices, it doesn't go away. The diamond symbol thingy that appears when I use my tablet's pen doesn't either. I've been without a way to do digital art properly for about a week now, and I have a few things I'd like to finish, so I'm starting to freak out a little. I've tried restarting, shutting down, changing the line weight/stabilizer, changing brushes, reinstalling krita, etc. and nothing seems to be working. The only thing that sort of works is zooming in super close to the line you draw, but even then it's still jagged and pixelated-looking, just on a smaller scale.

What type of tablet do you have? I am not sure what you mean by a diamond, but the cursor sometimes changes if you are using a tablet. If it is a new laptop, there is a chance you might need to download the latest drivers from the tablet website. That fixes quite a few issues for other people.

If that doesn't work, you could try to download an older version to see if it is related to a new update?
https://download.kde.org/stable/krita/