If You accidentally changed DPI to very high value and cannot revert to previous settings from GUI, use command line:

$ gsettings list-schemas | grep mate| grep font
$ gsettings list-keys org.mate.font-rendering
rgba-order
antialiasing
dpi
hinting
$ gsettings get org.mate.font-rendering dpi
500.0
$ gsettings set org.mate.font-rendering dpi 96