You could definitely do this with the mapper. Does each row need to taper? Or can they be the same width all the way down? Either way it’s possible, it would just be more complex if they need to taper.
It might also just be easiest to use this: 2D Pixel coordinate mapping tool for images
That way a little bit of slop in how neatly the LEDs are laid out is reflected in the pattern, like if they don’t perfectly line up vertically.