You can render at 2x in Mate, and then scale it down slightly (ie 1.25x1.25) with xrandr, but taking a large image and scaling it down using a cubic filter won't look as sharp as real fractional scaling.

The command you gave is upscaling, which will be worse than 2x + downscaling.

Real fractional scaling scales the sizes of elements before rendering. This results in the sharpest image and there is no resizing/filtering in the loop.