Can Thumbnail quality be improved?

I think you need to go to the gallery settings and change the thumbnail width to something bigger. If your thumbnail width is smaller than the box it is in then it will be enlarged to fit and so not look as sharp.
Not sure how word press is handling it but its possible using med size images.

@landyvlad,

The reason why the FooGallery images do not look sharp is because they are being stretched slightly. This is unfortunately how the justified gallery calculates it’s layout to fit all the images in.
The reason why the built-in WP gallery images look clear is because the thumb images are very large and are being reduced in size to fit on the page. This might look nice and sharp, but can also not be ideal as the page will take longer to load.

A few suggestions to help make the thumbs look sharper:

1) Goto FooGallery Settings and under images, set the Thumbnail image quality to a higher value, even 100.
2) As @generalnumpty suggested, edit your gallery and play with the row height and max row height values of your justified gallery. The justified gallery stretches the images slightly in order to get them to fit nicely.
3) You can try to change to the responsive gallery and set the exact size of the thumbs. This gallery template does not stretch images to fit.
4) Under the Retina support metabox, check 2x which will create a higher res thumb and show that version if the browser can handle it.

Try those and see if you get a better result

Thank you, will do ??