We introduced quite some improvements in the way Modeler is rendered on High DPI screens. Please try the latest release (at the moment of writing latest is 6.9.0, here are the release notes) and let us know whether it improves the experience for you.
Have you tried changing your shortcut settings:
For example running in compatibility mode or enabling : "Disable display scaling on high DPI settings"? Not sure if it works but it initially worked for me when I moved to a 4K display and MX had some issues with that. (even though it is not perfect yet)
And this post: http://www.danantonielli.com/adobe-app-scaling-on-high-dpi-displays-fix/ gives another sollution you could try.
Had same issue with 7.22.2 version of desktop modeler and i resolved it by changes my compatibility settings on the Mendix shortcut: