This is happening because mIRC is not marked as a DPI aware application. In order for applications to be DPI aware, they need to make sure that all interface elements are correctly scaled (positioned and sized) when non-standard DPI settings are used.
As far as I can tell, mIRC seems to display just fine when marked as DPI aware when non-standard DPI settings are used, although there a few minor display issues. I will look into marking mIRC as DPI aware by default for the next version.
For now, you can prevent the fuzzy font using this method: right-click on the mIRC icon on your Windows desktop, select Properties, Compatibility, and check the "Disable display scaling on high DPI settings" option. This will make Windows 7 display mIRC without any scaling, which will result in a clearer font in all windows and dialogs.