That’s not a KDE specific issue, it’s Qt defaulting to the default ugly old Plastic theme.
KDE uses Qt, but unlike GTK, Qt isn’t made for KDE. Qt have their own defaults that are generic and doesn’t make any assumption about running on KDE or Gnome. And Gnome doesn’t bother providing any configurations to Qt, because they live in their own GTK bubble where nothing else matters. Adwaita just happens to be the default GTK theme regardless.
Ideally, your distro would take care of setting a saner default Qt theme. There’s even a Breeze theme for GTK, so they could set it to that and everything would look nice and uniform.