I am using kde neon 5.8.4 user edition. When I press switch windows ALT+TAB, the icons that appear are distorted most of times. I have described the issue in detail here: http://askubuntu.com/questions/864056/low-resolution-icons-in-task-switcher-in-kde
KWin uses the icon provided by the window. Unfortunately many applications do not provide high resolution icons. In that case KWin cannot do anything about it. Starting with Plasma 5.9 KWin will also use icons from the .desktop file as the task manager, if the information is available. Unfortunately we have to expect that in the cases where there is no high resolution icon installed on the window, we won't have the desktop file either. overall not much the window manager can do.