Is ubuntu mate supporting the Raspberry PI 3 model B+ yet?
No. Release was originally planned for the mid of july. There are no news from the developers since then, so I think this project is dead.
Do you especially need the Mate desktop? You'll find armhf and arm64 versions of Xubuntu 18.04 for the Raspberry Pi on this thread https://ubuntu-mate.community/t/aarch64-on-raspberry-pi-2-rev-1-2-3b-3b/16853 . The Pi 3B+ works out of the box.
If you really want Mate, then there are multiple threads on here that explain how to get it on the 3B+. No Mate image for 18.04 exists (yet), but you can upgrade from 16.04.