I've installed Ubuntu in a Macbook Pro Retina. It looks terrible, though, even after adjusting the resolution to 2880x1800:
Why this is happening? I have already enabled Retina resolution on video configs of parallels.
On my 2015 MacBook Pro 15" with a Retina Screen, I had to adjust the resolution to the highest (native) setting and then set the scale to 200%:
gnome-tweak-tool
and try different anti aliasing modes