libadwaita is the best thing happened in the Linux ecosystem. This way application developers don’t have to care about theming, as it is fixed to one thing. Sure there will be the dark mode, but this is the only thing they have to take care of and take a look. When colouring is added, adwaita isn’t as bad as everybody thinks it is. pamac was always being designed as a Gnome app and always follows the latest design concepts - period.
If you don’t like it, you may want to use Octopi, which is a pacman UI wrapper.