Is the there a KiCAD plugin for making automatic board outlines?
Well, rather than a plug in, you can use an external graphics package and create the PCB outline there.  I use Inkscape, export the image via a .dxf file and then import as a graphic. Others might use Adobe Illustrator, and there are even more others than can export .dxf. Obviously AutoCad/SolidWorks and their derivatives/competitors.
So do this:-

To produce this:-

A point to note is that no matter how good a plug-in would be, it would still be a functional compromise when compared to a dedicated CAD/graphics package. I've had boards manufactured via this process and it's perfectly accurate. Just don't get too creative because you have to bear in mind the manufacturing capability of the board houses. They usually have a capabilities page somewhere.