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 AArch64 on Raspberry Pi 2 (rev 1.2), 3B, 3B+ . 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.
There’s always the method I describe in “How-To Set Up and Upgrade Ubuntu MATE 16.04 on a Pi 2/3/3B+”