.app bundle displaying low resolution Tkinter window
Found the answer by carefully reading the docs.

I was missing High Resolution retina settings in the app part of my spec file, as mentioned in the above mentioned docs: 

For example, when you use PyQt5, you can set NSHighResolutionCapable to True to let your app also work in retina screen

Once I added the info_plist parameter as provided below the problem was gone:) 

app = BUNDLE(exe,
     name='myscript.app',
     icon=None,
     bundle_identifier=None
     info_plist={
        'NSHighResolutionCapable': 'True'
        },
     )