The dots are actually just an image file so you can technically make it whatever you want, but as memory serves if you make the menu window too tall it doesn’t run all the way down. But anyway… the file for the border is “/usr/lib/ubuntu-mate/mate-menu/dotted.png” … I just copied it and cleared everything out of the image and made saved it with nothing in it, so it’s basically a 0 opacity blank file.
@Robsteady Can I be lazy and ask that you to attach the Ubuntu MATE logo you have made for MATE Menu and also the dotless dots image. I’d like to bundle them in MATE Menu