i’ve run into the same problem a couple weeks ago. already did a post at the time.
the bad quality jpeg issue ocurred after updating to kirby 3.9 and only happens on the webserver. the local project is running fine with mamp. even thumbs in the panel don’t look good.
as a workaround i’ve changed the format for thumbs to gif in the config. not really a good solution but at least images were not pixelated anymore.