Why is my top screen showing “glitching” out?

Looks like there’s a bad connection to the top screen. Could be a loose or damaged cable or just a bad screen. Its worth noting that based on the videos, this is a New 3ds XL, not the older 3ds xl. The relevant repair guide can be found here. If you are comfortable with it and have the tools, you could first try checking where the ribbon cables to make sure they have a good connection. If they do you might be looking at needing a new screen.