The QT5 version of the Muon software suite 5.2.1 is now available in the Manjaro Community repository for anyone interested.
However, because of the blushell repositories taking priority, the older 2.1.2 version in this repository will be installed by default instead. In order to install the new 5.2.1 version of Muon you will need to use the command line:
sudo pacman -S community/muon
PS. You could alternatively disable the blueshell repository or move it to the bottom of the repository list in pacman.conf file to solve these kinds of issues.
Please note that Muon uses packagekit and appstream on the back end, so it does not have AUR support ATM.