It looks like the lxpanel temperature monitor simply reads /sys/class/thermal/thermal_zone0/temp (from https://raspberrypi.stackexchange.com/questions/12765/change-temperature-monitor-to-display-in-fahrenheit . Source code: https://github.com/raspberrypi-ui/lxpanel/blob/master/plugins/thermal/thermal.c ).
If the mate applet can’t do this (and I haven’t used it in years) then it wouldn’t be hard to write a custom plugin https://github.com/mate-desktop/mate-sensors-applet/tree/master/plugins . This would be a nice little project for somebody. Essentially you would take an existing plugin like smu-sys and adapt it for your needs.
If that is too hard then an alternative is Raspberry Pi 2 Sensor