I assume this is already known (who could overlook it?). Still opening this report for sake of completeness. Application logos should be SVG files or of high resolutions. Some logos are also outdated (Dolphin for example). Reproducible: Always
First step is to ask your distribution to offer good quality icons, make sure they're installed. WRT dolphin, that's another problem to solve. Since they advertise Icon=system-file-manager, it's up to the generator to decide which icon to use.
Make sure your distribution installs the big icon sets, report to your distributor if it's not fixed.