My Surface is work on 150% DPI , The App looks unclear.
This is certainly the fix to use to fix the resolution.
You can permanently influence this setting by the following:-
 - Right click the Fiddler icon
 - Select Properties
 - 'Shortcut' tab
 - Add the text '-dpiaware' to the end of the target string per the below screen snap:-
 - Launch the program using the icon, voila!
After add the -dpiaware parameter, display is garbled, windows 10 with 4k dispaly
Why isn't -dpiAware on be default?
Did that solve the problem for you?
argument to Fiddler's command line, either by modifying the shortcut or by hitting Windows+R and typing fiddler.exe -dpiAware