There have been a number of DPI-related changes to improve support for scaling, multiple monitors, and so on, over the last few versions. In your case, it looks like the menubar has been scaled up, which is why the font is fuzzy.
mIRC currently uses <dpiAware>true</dpiAware> in the manifest file to enable DPI awareness, however it seems that for multiple monitors, an application needs to use <dpiAware>true/pm</dpiAware> for per monitor DPI awareness. I have added this to the next version. That said, it is not clear what the side-effects of this will be.
There is also a newer <dpiAwareness> element for Windows 10 that supports a "permonitorv2" option, however when I tested that under Windows 10, it caused MDI windows to be drawn incorrectly, so it does not look like it is an MDI-ready solution at this time.