Apparently not. Command
echo $QT_QPA_PLATFORMTHEME returns nothing.
EDIT: I was wrong. Command
echo $QT_QPA_PLATFORMTHEME returns
qt5ct and if I run qt5ct from terminal then it works fine. But when I run it from the mate control center then it throws the error.
qt5ct.desktop from caja works fine. Also, running qt5ct form the main MATE menu works fine (brisk menu doesn't show control center's items yet). So the problem only exists when I run it from the control-mate-center or from the advanced MATE menu, so maybe there is a bug at those making them to ignore the environmental variables and it needs to be reported on MATE devs?